The question of interest is no longer whether human social behavior is genetically determined; it is to what extent.
—Edward O. Wilson, 1978
Heredity is what sets the parents of a teenager wondering about each other.
—Laurence J. Peter
Studies of families and twins strongly suggest genetic influences on the development and expression of specific behaviors, but there is no conclusive research demonstrating that genes determine behaviors. In "The Interplay of Nature, Nurture, and Developmental Influences: The Challenge Ahead for Mental Health" (Archives of General Psychiatry, vol. 59, no. 11, November 2002), psychiatrist Michael Rutter observed that a range of mental health disorders from autism and schizophrenia to attention deficit hyperactivity disorder (ADHD) involve at least indirect genetic effects, with heritability ranging from 20 to 50%. He further asserted that genetically influenced behaviors also bring about gene-environment correlations.
Rutter explained the mechanism of genetic influence on behavior—genes affect proteins, and through the effects of these proteins on the functioning of the brain there are resultant effects on behavior. Rutter viewed environmental
influences as comparable to genetic influences in that they are strong and pervasive but do not determine behaviors, and studies of environmental effects show that there are individual differences in response. Some individuals are severely affected and others experience few repercussions from environmental factors. This has given rise to the idea of varying degrees of resiliency—that people vary in their relative resistance to the harmful effects of psychosocial adversity—as well as the premise that genetics may offer protective effects from certain environmental influences.
Jan Strelau, in "The Contribution of Genetics to Psychology Today and in the Decade to Follow" (European Psychologist, vol. 6, no. 4, December 2001), asserted that the proportion of phenotypic variance that may be attributed to genetic variance shows that personality traits, including temperament as well as specific behaviors and intelligence, have a heritability ranging from 40% to about 60%, but that it is primarily environmental influences that explain individual differences. He also wrote that genetics influence the environment experienced by individuals, which explains how, for example, children growing up in the same family often experience and interpret their environments differently. This also explains why individuals who share the same genes though living apart show some concordance in selecting or creating similar experiences.
Traditional psychological theory holds that attitudes are learned and most strongly influenced by environment. In "The Heritability of Attitudes: A Study of Twins" (Journal of Personality and Social Psychology, vol. 80, no. 6, June 2001), James Olson et al. examined whether there is a genetic basis for attitudes by reviewing earlier studies and conducting original research on monozygotic and dizygotic twins. Olson and his colleagues argued that the premise that attitudes are learned is not incompatible with the idea that biological and genetic factors also influence attitudes. They hypothesized that genes probably influence predispositions or natural inclinations, which then shape environmental experiences in ways that increase the likelihood of the individual developing specific traits and attitudes. For example, children who are small for their age might be teased or taunted by other children more than their larger peers. As a result, these children might develop anxieties about social interaction, with consequences for their personalities such as shyness or low self-esteem discomfort with large groups.
In a twin study by Olson et al., nonshared environmental factors—unique experiences of each member of a twin pair—were the most powerful contributors to variability in thirty attitude factors, whereas shared environmental factors—experiences common to both members of a twin pair—contributed only slightly to variance in the thirty factors. There was also strong evidence that differences between respondents in many of their expressed attitudes were partly determined by genetic factors. Twenty-six of the thirty individual attitude items yielded significant genetic effects, and these effects were observed on a wide range of topics, from attitudes about organized religion and support for the death penalty to attitudes toward participating in organized sports and enjoyment of roller coaster rides. The investigators concluded that attitudes are learned but also depend on biological factors.
In a study supported by the NIH, Amy Abrahamson, Laura Baker, and Avshalom Caspi examined genetic influences on attitudes of adolescents and reported their findings in "Rebellious Teens? Genetic and Environmental Influences on the Social Attitudes of Adolescents [Personality Processes and Individual Differences]" (Journal of Personality and Social Psychology, vol. 83, no. 6, December 2002). The purpose of their study was to investigate sources of familial influence on adolescent social attitudes in an effort to understand whether and how families exert an influence on the attitudes of adolescents. They wanted to pinpoint the age when genetic influences actually emerge and to determine the extent to which parents and siblings shape teens' views about controversial issues. Abrahamson, Baker, and Caspi explored genetic and environmental influences in social attitudes in 654 adopted and nonadopted children and their biological and adoptive relatives in the Colorado Adoption Project. Conservatism and religious attitudes were measured in the children annually from ages twelve to fifteen and in the parents during the twelve-year-old visit.
The study found that both conservatism and religious attitudes are strongly influenced by shared-family environmental factors throughout adolescence. Familial resemblance for conservative attitudes arise from both genetic and common environmental factors, and familial influence on religious attitudes is almost entirely in response to shared family environmental factors. These findings are different from previous findings from twin studies, which suggest that genetic influence on social attitudes do not emerge until adulthood. In contrast, the Colorado Adoption Project study detects significant genetic influence in conservatism as early as age twelve, but finds no evidence of genetic influence on religious attitudes during adolescence. Abrahamson, Baker, and Caspi concluded that genetic factors exert an influence on social attitudes much earlier than previously indicated. For example, significant genetic influences on variations in conservatism are identified as early as age twelve. The study provides further evidence that shared environmental factors contribute significantly to individual differences in social attitudes during adolescence.
Psychologist David Cohen downplayed environmental influences by specifically discounting parents' responsibility for mental illness and emotional problems in their
children. In Stranger in the Nest: Do Parents Really Shape Their Child's Personality, Intelligence, or Character? (New York: John Wiley and Sons, 1999), Cohen asserted that "good parenting" cannot overcome "bad genes" and that it is impossible to separate genetic background from environmental influence. Making a strong case for genetic influence, Cohen wrote, "The truth of the matter is that, if sufficiently strong, inborn potentials can trump parental influence, no matter how positive or negative. Some traits manifest themselves in such unexpected and uncontrollable ways that, for better or for worse, one's child may indeed seem like a perfect stranger."