Developmental stability and human violence.

Developmental stability (the precision with which genotypes are translated into phenotypes under physically stressful developmental conditions), is a major source of phenotypic and behavioural variation, yet researchers have largely ignored its potential role in the ontogeny of individual propensities toward human aggression and violence. In this study, we measured fluctuating asymmetry of the body and administered aggression and fighting history questionnaires to 229 college students (139 female and 90 male undergraduates). Among males, but not females, fluctuating asymmetry correlated negatively and significantly with the participants' number of fights and propensity to escalate agonistic encounters to physical violence. Principal components analyses and scree tests suggested that two psychometric factors underlie observed correlations between self-report measures of aggressive tendencies. The first factor, 'aggressive negative affect', reflected verbal aggression and hostility toward others, while the second factor, 'self-assessed fighting ability', reflected physical violence and a tendency to win fights. The two factors correlated minimally. For both males and females, the second factor correlated with number of fights while the first factor did not. Fluctuating asymmetry did not significantly correlate with either factor for either sex, but for both sexes, psychometric intelligence (IQ) correlated positively with the first factor.

Fluctuating asymmetry and psychometric intelligence.

Little is known about the genetic nature of human psychometric intelligence (IQ), but it is widely assumed that IQ's heritability is at loci for intelligence per se. We present evidence consistent with a hypothesis that interindividual IQ differences are partly due to heritable vulnerabilities to environmental sources of developmental stress, an indirect genetic mechanism for the heritability of IQ. Using fluctuating asymmetry (FA) of the body (the asymmetry resulting from errors in the development of normally symmetrical bilateral traits under stressful conditions), we estimated the relative developmental instability of 112 undergraduates and administered to them Cattell's culture fair intelligence test (CFIT). A subsequent replication on 128 students was performed. In both samples, FA correlated negatively and significantly with CFIT scores. We propose two non-mutually exclusive physiological explanations for this correlation. First, external body FA may correlate negatively with the developmental integrity of the brain. Second, individual energy budget allocations and/or low metabolic efficiency in high-FA individuals may lower IQ scores. We review the data on IQ in light of our findings and conclude that improving developmental quality may increase average IQ in future generations.