Human Aggression Across the Lifespan

May combine two claims.

About half of the variance in aggressive behavior is explained by genetic influences in both males and females, with the remaining half explained by nonshared environmental factors.

“These studies together show that about half (50%) of the variance in aggressive behavior is explained by genetic influences in both males and females, with the remaining 50% of the variance being explained by environmental factors not shared by family members.”

• Also stated in: § I. Heritability of Aggression: Twin and Adoption Studies · plain

  “In one early meta-analysis of 24 twin and adoption studies, heritable influences explained about half of the total variance in aggressive behavior and the nonshared environment explained the remaining 50% (Miles and Carey, 1997).”

• cites Miles and Carey, 1997 sourceNot checked yet

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• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Neither study design (twin vs. sibling adoption design) nor sex (male vs. female) impacts the magnitude of genetic and environmental influences on aggression.

“Neither study design (twin vs. sibling adoption design) nor sex (male vs. female) seems to impact the magnitude of the genetic and environmental influences on aggression.”

May combine two claims.

The heritability of aggressive behavior based on teacher ratings is 54%, shared environmental influences account for 6%, and the nonshared environment accounts for the remaining 40% of the variance.

“The heritability of aggressive behavior based on teacher ratings is 54%, shared environmental influences account for 6% [c2: 2rDZ–rMZ=2(0.33–0.60)], and the nonshared environment accounts for the remaining 40% of the variance.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Both MZ and DZ correlations decline steadily across development, suggesting the waning importance of shared environmental effects from childhood to adolescence and then adulthood.

“However, both MZ and DZ correlations decline steadily across development, suggesting the waning importance of shared environmental effects from childhood to adolescence and then adulthood.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

For reactive and proactive aggression in 9-10-year-old boys, only genetic influences were important for self-reports, but both genes and shared environment were important for teacher and parent ratings.

“A similar pattern was found for reactive and proactive aggression in 9–10-year-old boys, whereby only genetic influences were important for self-reports, but both genes and shared environment were important for teacher and parent ratings (Baker et al., 2008).”

• cites Baker et al., 2008Not checked yet

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According to twin correlations, the heritability of aggressive behavior based on laboratory observation is approximately 32%, dominant genetic effect accounts for approximately 10%, and the nonshared environment accounts for the remaining 58% of the variance.

“According to the twin correlations summarized in Fig. 8.2, the heritability of aggressive behavior based on laboratory observation is approximately 32% [h2: 2(rMZ–rDZ)=2(0.27–0.11)], dominant genetic effect accounts for approximately 10% [d2: 2(rMZ–rDZ)=2(0.27)–4(0.11)], and the nonshared environment accounts for the remaining 58% of the variance.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Aggressive behavior is clearly influenced by genetic factors across the lifespan, as MZ correlations exceed DZ correlations at all ages.

“These results show that aggressive behavior is clearly influenced by genetic factors across the lifespan, given the fact that the MZ correlations exceed those for DZ pairs at all ages.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

The heritability of aggressive behavior based on parent/caregiver ratings is 54%, shared environmental influences account for 17%, and the nonshared environment accounts for the remaining 29% of the variance.

“The heritability of aggressive behavior based on parent/caregiver ratings is 54%, shared environmental influences account for 17%, and the nonshared environment accounts for the remaining 29% of the variance.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Rhee and Waldman (2002) summarized 51 twin and adoption studies on criminal behavior, delinquency, psychopathy, conduct disorder, antisocial personality disorder, and aggressive behavior; genetic factors explained 41% of variance in antisocial behavior, shared environment 16%, nonshared environment 43%.

“Rhee and Waldman (2002) also summarized the results from 51 twin and adoption studies on criminal behavior, delinquency, psychopathy, conduct disorder, and antisocial personality disorder, as well as aggressive behavior, in children, adolescents, and adults. Genetic factors explained 41% of the variance in antisocial behavior, 16% was explained by shared environmental influences, and the remaining”

• cites Rhee and Waldman (2002) sourceNot checked yet

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• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

The DZ correlation exceeds half the value of the MZ correlation (taken as evidence for shared environment) only in early childhood, but not in later age groups.

“The DZ correlation exceeds half the value of the MZ correlation (taken as evidence for shared environment) only in early childhood, but not in later age groups.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

The average twin correlations across 18 studies, weighted by sample size, are similar for MZ males (0.66) and females (0.63) and for DZ same-sex males (0.42) and females (0.35), indicating no appreciable quantitative sex differences in aggressive behavior.

“The average twin correlations across these 18 studies, weighted by their respective sample sizes, shows quite similar twin correlations for both identical (rMZ–Males= 0.66; rMZ–Females= 0.63) and nonidentical same-sex pairs (rDZ–Males= 0.42; rDZ– Females= 0.35), indicating that there are no appreciable quantitative sex differences in aggressive behavior.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Only a small handful of studies have reported significant differences in heritability of aggression for males and females, primarily for younger samples.

“As indicated in Table 8.2, only a small handful of studies have reported significant differences in heritability of aggression for males and females (and these are primarily for younger samples; e.g., Hudziak et al., 2003; van Beijsterveldt et al., 2003; Vierikko et al., 2004).”

• cites Hudziak et al., 2003 sourceNot checked yet

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• cites van Beijsterveldt et al., 2003 sourceNot checked yet

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• cites Vierikko et al., 2004 sourceNot checked yet

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Both Burt (2009) and Rhee and Waldman (2002) examined nonadditive genetic effects, but only Rhee and Waldman (2002) found significant nonadditive genetic effects for antisocial behavior.

“Both Burt (2009) and Rhee and Waldman (2002) examined nonadditive genetic effects, but only Rhee and Waldman (2002) found significant nonadditive genetic effects for antisocial behavior.”

• cites Burt (2009) sourceNot checked yet

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• cites Rhee and Waldman (2002) sourceNot checked yet

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There is no evidence of qualitative sex differences, as the weighted twin correlation for opposite-sex DZ pairs (0.38) is similar to same-sex DZ twin correlations (0.42 in males and 0.35 in females).

“There is no evidence of qualitative sex differences either, given that the weighted twin correlation for MF (DZMale–Female) is 0.38, which is quite similar to the same-sex DZ twin correlations (0.42 in males and 0.35 in females).”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

The heritability of aggressive behavior based on self-reports is 40% and the nonshared environment accounts for the remaining 60% of the variance, with no evidence of shared environmental contribution.

“The heritability of aggressive behavior based on self-reports is 40% and the nonshared environment accounts for the remaining 60% of the variance. There is no evidence of shared environmental contribution, as the DZ correlation is approximately half the MZ correlation.”

• AssumptionThe authors state this as an assumption: Monozygotic twins share 100% of their genes.
• AssumptionThe authors state this as an assumption: Dizygotic twins share on average 50% of their genes.

May combine two claims.

Aggressive behavior in childhood is influenced by genetic factors in all studies, and most of these studies also report shared environmental influences.

“Aggressive behavior in childhood is influenced by genetic factors in all studies, and most of these studies also report shared environmental influences (Table 8.2; e.g., Baker et al., 2008; Eley et al., 1999; Hudziak et al., 2003; Schmitz et al., 1995; Simonoff et al., 1998; Tuvblad et al., 2009; van den Oord et al., 1996; Vierikko et al., 2004; but for an exception see Dionne et al., 2003; Taylor”

• cites Baker et al., 2008Not checked yet

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• cites Eley et al., 1999 sourceNot checked yet

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• cites Hudziak et al., 2003 sourceNot checked yet

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May combine two claims.

In one longitudinal analysis, the stability in reactive aggression from childhood to adolescence could be explained by genetic (48%), shared (11%), and nonshared (41%) environmental influences, whereas the continuity in proactive aggression was primarily genetically (85%) mediated.

“In one of the few longitudinal analyses of these constructs, the stability in reactive aggression from childhood to adolescence could be explained by genetic (48%), shared (11%), and nonshared (41%) environmental influences, whereas the continuity in proactive aggression was primarily genetically (85%) mediated (Tuvblad et al., 2009).”

• cites Tuvblad et al., 2009 sourceNot checked yet

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May combine two claims.

There are a handful of adoption studies and over two dozen twin studies that have specifically examined genetic and environmental influence in aggression in nonselected samples from Northern America and Europe that are reasonably representative of the general population.

“Nonetheless, there are a handful of adoption studies and over two dozen studies of twins raised together which have specifically examined the genetic and environmental influence in aggression in nonselected samples from Northern America and Europe that are reasonably representative of the general population.”

May combine two claims.

Reactive and proactive aggression have each been found to be mainly influenced by genetic and nonshared environmental factors, and their genetic correlation is significantly less than 1.0, indicating some genetic specificity for the two forms.

“Although reactive and proactive aggression have each been found to be mainly influenced by genetic and nonshared environmental factors, their genetic correlation is significantly less than 1.0, indicating some genetic specificity for the two forms of aggression.”

• cites Baker et al., 2008Not checked yet

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• cites Tuvblad et al., 2009 sourceNot checked yet

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May combine two claims.

Significant shared environmental effects were not found for either parent or teacher ratings, and were therefore dropped from the models, suggesting that any shared environmental influences are likely to be included in the genetic component.

“Significant shared environmental effects were not found for either parent or teacher ratings, and were therefore dropped from the models, suggesting that any shared environmental influences are likely to be included in the genetic component.”

• cites Haberstick et al., 2006aNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

The CBCL has been used more often than any other single instrument in behavioral genetic studies of aggression.

“By and large, the Child Behavior Checklist (CBCL; Achenbach, 1991b) has been used more often than any other single instrument in behavioral genetic studies of aggression.”

• cites Achenbach, 1991bNot checked yet

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May combine two claims.

Neither study design (twin vs. sibling adoption design) nor sex impacts the nature or magnitude of genetic and environmental influences on aggression.

“Neither study design (twin vs. sibling adoption design) nor sex, on the other hand, seems to impact the nature or magnitude of these genetic and environmental influences on aggression.”

May combine two claims.

Form of aggression, method of assessment, and age of subjects are significant moderators of the magnitude of genetic and environmental influences on aggressive behavior.

“Form of aggression (reactive, proactive, direct/physical, indirect/relational), method of assessment (observation, self, teacher, parent/caregiver), and age of the subjects—all seem to be significant moderators of the magnitude of genetic and environmental influences on aggressive behavior.”

May combine two claims.

Twin and adoption studies and meta-analyses show that about half of the variance in aggressive behavior is explained by genetic influences in both males and females, with the remaining half explained by nonshared environmental factors.

“Studies (and meta-analyses) including both twin and adoption samples show that about half (50%) of the variance in aggressive behavior is explained by genetic influences in both males and females, with the remaining 50% of the variance being explained by nonshared environmental factors.”

May combine two claims. The quote only partly states this claim.

Not all individuals are affected to the same degree by experiences and exposures, and genetic predispositions may have different effects depending on the environment.

“Our view is that while many, if not most, humans may have the potential for aggression and violence under the right circumstances, not all individuals will succumb to these behaviors under the same circumstances.”

May combine two claims.

In Milgram's first set of experiments, 65% (26 of 40) of participants complied with the instruction to administer what they believed to be a final, massive 450-volt shock, while the remaining 35% did not comply.

“In Milgram's (1963) first set of experiments, while 65% (26 of 40) of participants complied with the instruction to administer what they believed to be a final, massive 450-volt shock, the remaining 35% did not comply.”

• cites Milgram, 1963 sourceNot checked yet

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Classified as probably not a substantial claim.

It is difficult to discern whether the parent/caregiver rating patterns reflect true shared environment or are instead an artifact of rater bias whereby raters are less able to discriminate between the two twins' aggressive behavior and thus inflate the similarities between them, regardless of zygosity.

“It is difficult to discern whether the parent/caregiver rating patterns reflect true shared environment or are instead an artifact of rater bias whereby raters are less able to discriminate between the two twins' aggressive behavior and thus inflate the similarities between them, regardless of zygosity.”

Classified as probably not a substantial claim.

Increasing heritability estimates from child to adulthood could also be explained by different methods of assessment, such as parental bias leading to overestimation of shared environmental effects and thus attenuating heritability estimates in childhood.

“Increasing heritability estimates from child to adulthood could therefore also be explained by different methods of assessment as well (e.g., parental bias may lead to overestimation of shared environmental effects and thus attenuate heritability estimates in childhood).”

May combine two claims.

The overall pattern across studies indicates that genetic influences for aggressive behavior become increasingly more important, while shared environmental effects become less so as children develop from childhood, through adolescence, and into adulthood.

“The overall pattern across the studies presented in Table 8.2 and Fig. 8.1 indicates that genetic influences for aggressive behavior become increasingly more important, while shared environmental effects become less so as children develop from childhood, through adolescence, and into adulthood.”

May combine two claims.

Gene by environment (G × E) interactions are likely to exist, and the genetic propensity for aggression should exert its effects more strongly in some situations than others.

“Although few behavioral genetic studies have explicitly examined the question of gene by environment (G × E) interactions, we contend that such interactions are likely to exist and that the genetic propensity for aggression should exert its effects more strongly in some situations than others.”

• cites Cloninger et al., 1982 sourceNot checked yet

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May combine two claims.

The pattern of decrease in shared environment and concomitant increase in heritability during development is relatively common for personality traits and cognitive abilities, and has also been found for prosocial behavior.

“This pattern of decrease in shared environment, and a concomitant increase in heritability during development, is relatively common for personality traits and cognitive abilities (Bartels et al., 2002; Loehlin, 1992; Plomin et al., 2001; Scarr and McCartney, 1983), and has also been found for other phenotypes including prosocial behavior (Knafo and Plomin, 2006).”

• cites Bartels et al., 2002 sourceNot checked yet

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• cites Loehlin, 1992Not checked yet

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• cites Plomin et al., 2001Not checked yet

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May combine two claims.

Many participants who engaged in the aggressive behavior in Milgram's experiments stated they were very uncomfortable, and every participant reportedly questioned the experiment at some point or refused money promised for participation.

“Many of those who engaged in the aggressive behavior stated they were very uncomfortable doing so, and every participant reportedly questioned the experiment at some point or refused money promised for their study participation (Milgram, 1963).”

• cites Milgram, 1963 sourceNot checked yet

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May combine two claims.

Although the Milgram and Zimbardo studies provide clear evidence for the role of environment and social situations in affecting aggressive behavior, there are large individual differences in the propensity for violence and aggression even under extreme circumstances.

“Although the studies by Milgram and Zimbardo provide clear evidence for the role of environment and social situations in affecting aggressive behavior, there are, nonetheless, large individual differences in the propensity for violence and aggression, even under these extreme circumstances.”

• cites Milgram, 1963 sourceNot checked yet

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• cites Haney et al., 1973Not checked yet

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May combine two claims.

While many, if not most, humans may have the potential for aggression and violence under the right circumstances, not all individuals will succumb to these behaviors under the same circumstances.

“Our view is that while many, if not most, humans may have the potential for aggression and violence under the right circumstances, not all individuals will succumb to these behaviors under the same circumstances.”

May combine two claims.

A teacher is more likely to compare a child's behavior to his or her peers, whereas a parent is likely to compare a child's behavior to his or her siblings.

“A teacher is also more likely to compare a child's behavior to his or her peers, whereas a parent is likely to compare a child's behavior to his or her siblings (Bartels et al., 2003).”

• cites Bartels et al., 2003 sourceNot checked yet

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May combine two claims.

If the equal environments assumption is violated, higher correlations among MZ twins may be due to environmental factors rather than genetic factors, and heritability estimates will be overestimated.

“If this assumption is violated, higher correlations among MZ twins may be due to environmental factors, rather than genetic factors, and heritability estimate will be overestimated (Plomin et al., 2001).”

• cites Plomin et al., 2001Not checked yet

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• AssumptionThe authors state this as an assumption: Environmentally caused similarity is roughly equal for MZ and DZ twins.

May combine two claims.

It is likely that there are different etiologies for different forms of aggression; for example, defensive reactions to threatening stimuli may be more environmentally influenced, while more planned, proactive forms may be more genetically influenced.

“It is likely that there are different etiologies for different forms of aggression; for example, defensive reactions to threatening stimuli may be more environmentally influenced, while more planned, proactive forms may be more genetically influenced (Tuvblad et al., 2009).”

• cites Tuvblad et al., 2009 sourceNot checked yet

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May combine two claims.

Cates et al. (1993) found no genetic influences for assault but strong heritability for verbal hostility at 78% and indirect hostility at 70%.

“Yet, Cates et al. (1993) found no genetic influences for assault, but strong heritability for both verbal hostility (78%) and indirect hostility (70%).”

• cites Cates et al., 1993 sourceNot checked yet

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May combine two claims.

Studies using multifactorial aggression measures such as the BDHI suggest variability in heritability estimates across subscales, although the patterns are not entirely consistent across studies.

“Other studies based on multifactorial measures of aggression, such as the BDHI, suggest some variability in the heritability estimates across subscales, although the patterns are not entirely consistent across studies.”

• cites Coccaro et al., 1997 sourceNot checked yet

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• cites Cates et al., 1993 sourceNot checked yet

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May combine two claims.

Adoptive parents tend to be in good health and from higher socioeconomic levels, so adoption study findings may not always generalize to the entire population.

“In terms of generalizability, it is often the case that adoptive parents tend to be in good health and from higher socioeconomic levels; thus, findings from adoption studies may not always be unquestionably generalized to the entire population (Rutter, 2006).”

• cites Rutter, 2006Not checked yet

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May combine two claims.

The less-than-perfect genetic correlation between reactive and proactive aggression, their different developmental etiologies, and the significant shared environmental effects found only in relational aggression support at least some genetic and environmental etiological distinction among different forms of aggression.

“Together these findings—that is, the less than perfect genetic correlation between reactive and proactive forms, their different developmental etiologies, and the significant shared environmental effects in relational aggression only— provide support for at least some genetic and environmental etiological distinction among different forms of aggression.”

May combine two claims.

Genetic factors influence relational aggression, with heritability estimates of 49% in self-report and 42% in boys and 21% in girls in parental reports.

“Like reactive and proactive forms, relational aggression appears to be influenced by genetic factors, both in self-report (49%) as well as parental reports (boys, 42%; girls, 21%).”

• cites Tackett et al., 2009 sourceNot checked yet

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May combine two claims.

Shared environmental influences affect relational aggression only in parental reports, with estimates of 22% for boys and 46% for girls, and not in self-report measures.

“However, unlike the other more direct forms of aggression, relational aggression is also influenced by shared environmental influences, but only in parental reports (boys, 22%; girls, 46%) and not in self-report measures (Tackett et al., 2009).”

• cites Tackett et al., 2009 sourceNot checked yet

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May combine two claims.

Twins and singletons have similar rates of psychiatric disorders such as ADHD, oppositional defiant disorder, and conduct disorder, and similar behavioral and emotional problems.

“Twins and singletons have been found to experience similar rates of psychiatric disorders (e.g., attention-deficit hyperactivity disorder (ADHD), oppositional defiant disorder, conduct disorder) and behavioral and emotional problems (Gjone and Novik, 1995; Moilanen et al., 1999; Simonoff et al., 1997; van den Oord et al., 1995).”

• cites Gjone and Novik, 1995Not checked yet

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• cites Moilanen et al., 1999 sourceNot checked yet

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• cites Simonoff et al., 1997 sourceNot checked yet

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Probably not a substantial claim: may not pass S4 (based).

If MZ twins are more similar than DZ twins, it may be inferred that the difference is caused by genetic effects.

“If MZ twins are more similar than DZ twins, it may be inferred that the difference is caused by genetic effects.”

• AssumptionThe authors state this as an assumption: Environmentally caused similarity is roughly equal for MZ and DZ twins.
• AssumptionThe authors state this as an assumption: MZ twins share 100% of their genes and DZ twins on average share 50% of their genes.

May combine two claims.

A meta-analysis on antisocial and aggressive behavior found no differences between twin and sibling adoption studies, with h2 = 48%, c2 = 13%, and e2 = 0.39%.

“This is also well in line with the results of a meta-analysis on antisocial andaggressive behavior that foundnodifferences between twin and siblingadoption studies (h2 = 48%; c2 = 13%, e2 = 0.39%) (Rhee and Waldman, 2002).”

• cites Rhee and Waldman, 2002 sourceNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Not all twin studies have examined sex-limited effects, either qualitative or quantitative, and several studies combined males and females when computing twin correlations, making it impossible to evaluate these effects based on published results shown in Table 8.2.

“Not all twin studies have examined sex-limited effects, either qualitative or quantitative, and several studies combined males and females when computing twin correlations, making it impossible to evaluate these effects based on published results shown in Table 8.2.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Relational aggression, which involves social manipulation such as gossip and peer exclusion, is often more indirect than other forms of aggression.

“Relational forms of aggression, which involve social manipulation such as gossip and peer exclusion, are often more indirect compared to other forms of aggression (Crick and Grotpeter, 1996).”

• cites Crick and Grotpeter, 1996Not checked yet

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May combine two claims.

Genetic influences are generally found for most, if not all, forms of aggression, although somewhat different genetic factors may operate across these forms.

“Overall, genetic influences are generally found for most, if not all forms of aggression, although somewhat different genetic factors may be operating across these different forms.”

May combine two claims.

In the Deater-Deckard and Plomin (1999) study, adopted children had higher aggression scores than nonadopted children, consistent with adoptees having higher genetic risk for aggression.

“In the Deater-Deckard and Plomin (1999) study, the adopted children did in fact have higher aggression scores compared to the nonadopted children, consistent with the notion that adoptees may be at higher genetic risk for aggression compared to nonadopted individuals.”

• cites Deater-Deckard and Plomin, 1999Not checked yet

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May combine two claims.

Twins differ from singletons in having lower birth weight due to shorter gestation and delayed language development.

“There are, however, two ways in which twins differ from singletons: (i) lower birth weight, due to shorter length of gestation (Plomin et al., 2001) and (ii) delayed language development (Rutter and Redshaw, 1991).”

• cites Plomin et al., 2001Not checked yet

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• cites Rutter and Redshaw, 1991Not checked yet

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Probably not a substantial claim: may not pass S4 (based). May combine two claims.

Self-reporters are aware of their own motives and behaviors, which may go undetected by their caregivers, teachers, or peers.

“Self-reporters are aware of their own motives and behaviors, which may go undetected by their caregivers, teachers, or peers.”

Probably not a substantial claim: may not pass S2 (checkable), S3 (load-bearing), S4 (based).

Caregivers or teachers may be able to understand difficult and complex constructs better than children.

“On the other hand, caregivers or teachers may be able to understand difficult and complex constructs better than children.”

May combine two claims.

Multivariate genetic analyses in both studies found genetic correlations among BDHI subscales below unity, supporting genetic specificity for the subscales.

“In multivariate genetic analyses, both studies found some support for genetic specificity for the various subscales, similar to what has been found for reactive and proactive aggression, in that genetic correlations (rG) among the BDHI subscales were less than unity”

• cites Coccaro et al., 1997 sourceNot checked yet

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• cites Cates et al., 1993 sourceNot checked yet

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May combine two claims.

Children with birth complications are more likely to later develop antisocial and aggressive behavior, but birth complications may not by themselves predispose such behavior and require an environmental risk factor.

“However, studies have shown that children with birth complications are more likely to later develop antisocial and aggressive behavior (Raine, 2002), but birth complications may not by themselves predispose antisocial and aggressive behaviors, but will require the presence of an environmental risk factor (e.g., poor parenting, maternal rejection).”

• cites Raine, 2002 sourceNot checked yet

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May combine two claims.

Findings from moderated variance components studies show that heritable influences on aggressive behavior vary depending on environmental context, indicating the importance of environmental risk factors in the development of aggressive behavior and for gene expression.

“Findings from these studies show that heritable influences on aggressive behavior vary depending on environmental context, indicating the importance of the environmental risk factors in the development of aggressive behavior as well as for gene expression.”

May combine two claims.

In about 800 Swedish men adopted at an early age, when both inherited and environmental risk factors were present, 40% were criminal; with only genetic factors, 12.1% were criminal; with only environmental factors, 6.7% were criminal; and with neither, 2.7% were criminal.

“When both inherited factors and environmental risk factors were present, 40% were found to be criminal; if only genetic factors were present, 12.1% were criminal; if only environmental factors were present, 6.7% were criminal; and with neither inherited nor environmental factors being present, 2.7% were criminal (Cloninger et al., 1982).”

• cites Cloninger et al., 1982 sourceNot checked yet

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May combine two claims.

Genetic predispositions may be associated with certain social environments or experiences, leading to a correlation between genes and environments.

“It is possible, however, that some genetic predispositions may be associated with certain kinds of social environments or experiences, leading to a correlation between genes and environments.”

Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

Studies relying on variance partitioning often do not find significant gene-environment interaction effects, or find that they explain a very small portion of total variance, and such effects are dropped from further analysis.

“Studies relying on variance partitioning often do not find significant G × E effects, or find that they explain a very small portion of the total variance, and are thus dropped from further analysis.”

May combine two claims.

Evidence of gene-environment interaction in aggressive behavior has been reported in twin and adoption studies, and more recently in molecular genetic studies.

“Evidence of G × E interaction in aggressive behavior has been reported in twin and adoption studies, and more recently in molecular genetic studies.”

May combine two claims.

Most twin studies report little or no shared environmental influences on aggressive behavior, suggesting that random mating is of little importance for aggressive behavior.

“Most twin studies report finding little or no shared environmental influences on aggressive behavior, suggesting that random mating is of little importance for aggressive behavior.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Passive gene-environment correlation occurs when genes overlap between parents and offspring, for example when a child with aggressive parents inherits genetic susceptibility for aggression and also experiences an adverse rearing environment.

“Passive rGE occurs when genes overlap between parents and their offspring. For example, a child with aggressive parents inherits genetic susceptibility for aggression as well as experiences an adverse rearing environment.”

• cites Scarr and McCartney, 1983Not checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing).

It is possible that genetic influences themselves have been underestimated and could be larger than the 50% or so suggested by meta-analyses and the paper's summary.

“It is possible, on the other hand, that genetic influences themselves have been underestimated and could be larger than the 50% or so than these meta-analyses and our summary suggest.”

Probably not a substantial claim: may not pass S3 (load-bearing).

No genetically informative studies have examined violent video game exposure as a possible moderator of genetic influence on aggression.

“No genetically informative studies have examined violent video game exposure as a possible moderator of genetic influence on aggression, however, leaving this as an important area in need of study.”

May combine two claims.

In molecular genetic studies, both genes and environment are measured rather than inferred from correlations among family members, so gene-environment interactions can be tested in the general population without reliance on a twin or adoption design.

“In molecular genetic studies, both genes and environment are measured, rather than inferred from correlations among family members. G × E interactions can therefore be tested in the general population, that is, without necessary reliance on a twin or adoption design.”

May combine two claims.

In a sample of Swedish 16-17-year-old twins, heritability for antisocial and aggressive behavior was higher in more affluent neighborhoods (boys 37%; girls 69%) than in less advantaged neighborhoods (boys 1%; girls 61%), while shared environment was higher in less advantaged neighborhoods (boys 69%; girls 16%) than in better-off neighborhoods (boys 13%; girls 6%).

“In a sample of Swedish 16–17-year-old twins, heritability for antisocial and aggressive behavior was higher in the more affluent neighborhoods (boys, 37%; girls, 69%) compared to the less advantaged neighborhoods (boys, 1%, girls, 61%). Conversely, the shared environment was higher in the less advantaged neighborhoods (boys, 69%; girls, 16%) compared to better-off neighborhoods (boys, 13%; girls,”

• cites Tuvblad et al., 2006 sourceNot checked yet

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May combine two claims.

When gene-environment interaction is not taken into account in behavioral genetic studies, heritability estimates will tend to be biased, with the direction of bias depending on whether the moderating environmental influences are shared or nonshared.

“When G × E is not taken into account in behavioral genetic studies, heritability estimates will tend to be biased, although the direction of the bias depends on whether the moderating environmental influences are of the shared or nonshared variety.”

May combine two claims.

Taylor et al. (2000) found that parents of twins were correlated for retrospectively reported delinquency, with r = 0.23 in families of boys and r = 0.35 in families of girls, and concluded that assortative mating is modest.

“Taylor et al., 2000 found that parents of twins were correlated for retrospectively reported delinquency (r = 0.23 in families of boys and r = 0.35 in families of girls) and concluded that assortative mating is modest in degree.”

• cites Taylor et al., 2000 sourceNot checked yet

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May combine two claims.

The heritability of conduct problems was found to be lower in children growing up in dysfunctional families and higher in children growing up in families where dysfunction was absent.

“For example, the heritability of conduct problems was found to be lower in children growing up in dysfunctional families and higher in children growing up in families where dysfunction was absent (Button et al., 2005).”

• cites Button et al., 2005 sourceNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Only a few studies have examined the influence of gene-environment correlation on aggressive behavior, suggesting that more research is needed before firm conclusions can be drawn.

“Only a few studies have examined the influence of G × E correlation on aggressive behavior, suggesting that more research is needed on this topic before we can draw any firm conclusions.”

May combine two claims.

Genetic influence on aggressive behavior was found to be higher in schools with higher average levels of family warmth, while environmental influences were more important in schools with lower average levels of family warmth.

“Genetic influence on aggressive behavior was found to be higher in schools with higher average levels of family warmth. In contrast, environmental influences (both shared and nonshared) were more important in schools with lower average levels of family warmth (Rowe et al., 1999).”

• cites Rowe et al., 1999 sourceNot checked yet

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Classified as probably not a substantial claim.

The mixed findings in the literature suggest that more research is needed to resolve whether there is a link between exposure to violent video games and aggressive behavior.

“The mixed findings in the literature clearly suggest that more research is needed to resolve whether there is a link between exposure to violent video games and aggressive behavior.”

Probably not a substantial claim: may not pass S3 (load-bearing).

No behavioral genetic study has examined whether alcohol use moderates the influence of genetic and environmental factors on aggressive behavior.

“Despite this, there is so far no behavioral genetic study that had examined whether alcohol use moderates the influence of genetic and environmental factors on aggressive behavior.”

May combine two claims.

Nonrandom mating can increase resemblance for DZ but not MZ twin pairs, biasing shared environmental estimates upward and additive genetic effects downward.

“Assortative mating in the parent generation acts to increase the resemblance between dizygotic twins and thereby bias shared environmental estimates upward and additive genetic effects downward.”

• AssumptionThe authors state this as an assumption: Mating for aggression in the parents of twins is random.

May combine two claims.

A review including 130 independent studies found that alcohol was correlated with both criminal and domestic violence.

“For example, a review including 130 independent studies found that alcohol was correlated with both criminal and domestic violence (Lipsey et al., 1997).”

• cites Lipsey et al., 1997 sourceNot checked yet

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May combine two claims.

Few studies have examined the effects of gene-environment correlation in aggressive behavior, making it difficult to know the extent of their effect on heritability estimates in twin studies.

“Apart from these specific examples cited here, few studies have examined the effects of rGE in aggressive behavior, making it difficult to know the extent of their effect on heritability estimates in twin studies.”

May combine two claims.

Twins and singletons have been found to display similar scores on aggressive behavior, suggesting that findings from twin studies can be generalized to other populations.

“The EEA has been tested and found to hold for various phenotypes including aggressive behavior, and twins and singletons have been found to display similar scores on aggressive behavior, suggesting that findings from twin studies can be generalized to other populations.”

• AssumptionThe authors state this as an assumption: Twins and singletons display equal rates of aggressive behavior.

May combine two claims.

Findings from adoption studies should probably be generalized cautiously to other populations because adoptees tend to show higher scores on aggressive behavior compared to controls.

“In conclusion, findings from adoption studies should probably be generalized cautiously to other populations as adoptees tend to show higher scores on aggressive behavior compared to controls.”

May combine two claims.

A meta-analysis of 136 studies found that exposure to violent video games is a risk factor for increased aggressive behavior, aggressive cognition, and aggressive affect, and for decreased empathy and prosocial behavior.

“A recent meta-analysis that included 136 studies examined the effects of violent video games on aggressive behaviors. The evidence suggested that exposure to violent video games is a risk factor for increased aggressive behavior, aggressive cognition, and aggressive affect, and for decreased empathy and prosocial behavior.”

• cites Anderson et al., 2010 sourceNot checked yet

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May combine two claims.

The random mating assumption is probably violated for antisocial and aggressive behavior, as significant spouse correlations have been found in this domain.

“This assumption is probably violated when it comes to antisocial and aggressive behavior, as significant spouse correlations have been found suggesting that assortative mating exists in this behavioral domain (Krueger et al., 1998; Maes et al., 2007; Taylor et al., 2000).”

• cites Krueger et al., 1998 sourceNot checked yet

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• cites Maes et al., 2007 sourceNot checked yet

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• cites Taylor et al., 2000 sourceNot checked yet

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• AssumptionThe authors state this as an assumption: Mating for aggression in the parents of twins is random.

May combine two claims.

The relationship of violence and aggression with alcohol is well established, and some individuals become aggressive after consuming alcohol.

“It has long been known that some individuals become aggressive after consuming alcohol, and the relationship of violence and aggression with alcohol is well established (Bushman and Cooper, 1990; White et al., 2001).”

• cites Bushman and Cooper, 1990 sourceNot checked yet

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• cites White et al., 2001 sourceNot checked yet

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Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

Active gene-environment correlation is the process whereby an individual actively seeks environmental situations more closely matched to their genotype, and it has been suggested in adolescent drinking behavior, specifically among girls.

“Active rGE is defined as the process whereby an individual actively seeks out environmental situations that are more closely matched to the person's genotype. Active rGE has been suggested in adolescent drinking behavior, specifically among girls (Loehlin, 2010).”

• cites Loehlin, 2010 sourceNot checked yet

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May combine two claims.

These findings suggest that genetic effects are more likely to explain individual differences in aggression in more benign environments, whereas in more disadvantaged environments negative family-related factors and context-dependent risks may play a greater role than genetic predispositions.

“These findings suggest that genetic effects are more likely to explain individual differences in aggression in more benign environments, whereas in more disadvantaged environments negative family-related factors and context-dependent risks may play a greater role than genetic predispositions in aggressive and antisocial outcomes.”

May combine two claims.

Gene-environment interaction findings suggest that individuals are not all affected to the same degree by environmental exposures or by genetic predispositions.

“Findings from G × E studies on aggressive behavior suggest that not all individuals will be affected to the same degree by these environmental exposures, and also that not all individuals will be affected to the same degree by the genetic predispositions.”

May combine two claims.

There will always be individuals with the same combination of genetic risk and environmental vulnerability who do not engage in aggressive behavior, so risk is probabilistic rather than biologically deterministic.

“Yet, there will always be groups of individuals with the same combination of genetic risk and environmental vulnerability who will not engage in aggressive behavior. So, it is still only an increased (probabilistic) risk and not a biological determinism.”

Probably not a substantial claim: may not pass S3 (load-bearing).

Exactly how the genetic defect in MAO causes aggressive, impulsive behavior or mental retardation is not known.

“Although this genetic defect remains the first such link to aggressive behavior in humans, exactly how the genetic defect causes aggressive, impulsive behavior, or mental retardation is not known.”

May combine two claims.

The dopamine transporter gene DAT1 has been linked to ADHD and with violent behavior and delinquency in adolescents and young adults.

“Dopamine transporter gene DAT1 has also been linked to ADHD (Waldman et al., 1998), as well as with violent behavior and delinquency in adolescents and young adults (Guo et al., 2008).”

• cites Waldman et al., 1998 sourceNot checked yet

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• cites Guo et al., 2008Not checked yet

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May combine two claims.

Dopaminergic candidates including DRD4 have been found to be involved in ADHD and externalizing behavior, and DRD2 has been found to be involved in substance abuse and disinhibition.

“For example, dopaminergic candidates, including dopamine receptor DRD4, has been found to be involved in ADHD and externalizing behavior, and DRD2 has been found to be involved in substance abuse and disinhibition (Niv and Baker, 2010).”

• cites Niv and Baker, 2010Not checked yet

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May combine two claims.

The meta-analytic evidence strongly indicates that the MAO-A gene influences vulnerability to environmental stress and that this biological process can begin early in life.

“This provides strong evidence that the MAO-A gene influences vulnerability to environmental stress and that this biological process can be initiated early in life.”

May combine two claims.

There is at least one published failure to replicate the MAO-A finding, and the finding has not been replicated in females or in African Americans.

“However, there is at least one published failure to replicate (Haberstick et al., 2005), and this finding has been replicated neither in females (Sjöberg et al., 2007) nor in African Americans (Widom and Brzustowicz, 2006).”

• cites Haberstick et al., 2005 sourceNot checked yet

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• cites Sjöberg et al., 2007 sourceNot checked yet

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• cites Widom and Brzustowicz, 2006Not checked yet

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May combine two claims.

The DRD3 polymorphism has been found to be associated with impulsivity, with the association significant in violent but not nonviolent individuals, and no association between DRD3 and violence per se.

“The DRD3 polymorphism has been found to be associated with impulsivity. This association was significant in violent, but not in nonviolent individuals, and there were no association between DRD3 and violence per se (Retz et al., 2003).”

• cites Retz et al., 2003 sourceNot checked yet

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