The Scientific Fundamentalist

A look at the hard truths about human nature.

The 50-0-50 rule in action: Sociosexual orientation and risk of divorce

Do some people carry a genetic risk of divorce?

Another personality trait on which genes exert a very strong influence, and the effect of the shared environment is essentially zero, is sociosexual orientation (whether you are sexually promiscuous or not) and, as a consequence, the risk of divorce.

In an earlier post, I explain the 50-0-50 rule, the observation that, for many personality traits, roughly 50% of the variance between individuals is attributable to genes (heritability), roughly 0% of the variance is attributable to shared environment (parental socialization which affects all children in the family equally), and roughly 50% of the variance is attributable to nonshared environment (everything else that affects children from the same family differently, including the influences of peers).

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Evolutionary personality psychologists classify men and women on sociosexual orientation between the extremes of unrestricted and restricted.  Relative to sociosexually restricted individuals, sociosexually unrestricted individuals are more likely to:  1) engage in sex at an earlier point in their relationships; 2) engage in sex with more than one partner at a time; and 3) be involved in sexual relationships characterized by less investment, commitment, love, and dependency.  Sociosexual orientation (just like all other personality traits) is a relatively stable trait of individuals over the life course; in other words, people are either sociosexually restricted or unrestricted most of their lives.  While men in general are more unrestricted in sociosexual orientation than women, the variance within each sex is much greater than variance between the sexes.

A study of a large sample of Australian twins conducted by the great behavior geneticist J. Michael Bailey and colleagues shows that sociosexual orientation is another personality trait that roughly follows the 50-0-50 rule.  Their behavior genetic analysis shows that 49% of sociosexual orientation is heritable (determined by genes), 2% is attributable to shared environment, and 47% to unshared environment.  (Bailey et al.’s model attributes the remaining 2% of the variance to the respondent’s age, so it appears that sociosexual orientation changes very slightly over the life course).

As you can imagine, sociosexual orientation has a great impact on the risk of divorce.  Sociosexually unrestricted individuals are far more likely to experience divorce than sociosexually restricted individuals because they are more likely to engage in extramarital affairs.  As a result, the risk of divorce as an individual characteristic also follows the 50-0-50 rule.  It’s been known for a while that children of divorce face a greater likelihood of divorce themselves than children of intact marriages.  It appears that the most of the inheritance of divorce occurs through genetic transmission.

A study of twins conducted by Victor Jockin, Matt McGue, and David T. Lykken shows that, among men, 59% of the individual differences in the risk of divorce is heritable, 0% of the variance is attributable to shared environment, and 41% is attributable to unshared environment.  Among women, 55% is heritable, 0% is due to shared environment, and 45% is due to unshared environment.  The genetic effect in their study is so strong that the risk of divorce of an identical (MZ) twin can be predicted equally well by his or her co-twin’s characteristics as by his or her own characteristics.  Another study by McGue and Lykken shows that the breakdown for the risk of divorce is 52-0-48 for men and 53-0-47 for women.

Once again, it is remarkable how closely the breakdowns follow the 50-0-50 rule.  The close adherence to the rule is even more remarkable when you remember how different risk of divorce is from partisan attachment (how closely you identify with your political party).  Yet both as well as a wide array of other personality and individual traits follow the 50-0-50 rule of roughly half of their variance attributable to genes, none of their variance to shared environment of parental socialization, and the remaining half to unshared environment of experiences outside the home.  Parenting has virtually no intended effect on any of them.

Satoshi Kanazawa is an evolutionary psychologist at LSE and the coauthor (with the late Alan S. Miller) of Why Beautiful People Have More Daughters.

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