When we see children behaving like their parents, we often assume that the children learned the behavior from their parents. For example, if we see a boy with violent behavior, and we find out his father is also violent in many circumstances, we might assume that the boy learned violence from his father.
But correlations between the behavior of parent and child may reflect common genetic predispositions, not parental socialization. How can we differentiate the influence of genes from the influence of socialization?
Until fairly recently, there were basically three ways researchers could separate the effects of genes from the effects of parental socialization: adoption studies, twin studies, and (rarely) a combination adoption twin studies. Below, I review adoption studies and twin studies. I’ll get to the adopted twin studies a bit later.
In the most thorough adoption studies, researchers managed to collect data from kids who were adopted soon after birth, as well as data from the kids’ biological and adoptive parents. One could estimate the importance of genes by looking at whether adopted kids behaved more like their biological parents or more like their adoptive parents. One could estimate the importance of family socialization by looking at the similarity of adopted kids with their siblings who were biologically unrelated to them. All the variation in outcomes unaccounted for by genes and family socialization was attributed to extra-familial influences.
In a twin study, the researcher gathers data from pairs of twins. He ascertains whether the twins were born from one egg or two eggs. Twins born of one egg (monozygotic twins) are genetic clones. Twins born of two eggs (dizygotic twins) share half of their genes on average, although the exact amount shared varies from pair to pair.
In other words, dizygotic (DZ) twins share roughly half as much of their DNA as monozygotic (MZ) twins. Researchers exploit this fact in order to estimate how much variation in a trait is accounted for by variation in genes. According to the basic model underlying the so-called classic twin study, the variance in the trait accounted for by genes is equal to double the difference between the correlation between MZ twins and the correlation between DZ twins. Take IQ, for example. IQ scores of MZ cotwins correlate at around 0.8. DZ twins’ IQs correlate at around 0.4. Based on these numbers, we could say that genes account for 80% of variation in IQ. (0.8-0.4) × 2 = 0.8.