Psychologist Eric Turkheimer wrote The Three Laws of Behavior Genetics and What They Mean (in Current Directions in Psychological Science in 2000).

“First Law: All human behavioural traits are heritable.

Second Law: The effect of being raised in the same family is smaller than the effect of the genes.

Third Law: A substantial portion of the variation in complex human behavioural traits is not accounted for by the effects of genes or families.”

Steven Pinker examined these in his 2002 book The Blank Slate, as follows.

Heritability is the proportion of variance in a trait that corresponds to genetic differences. It can be studied in identical twins separated at birth and raised apart (100% shared genes, no shared environment), in identical twins raised together v fraternal twins raised together (half of the variable genes shared), and in biological siblings v adoptive siblings.

Identical twins reared together are more similar than fraternal twins raised together. Biological siblings are far more similar than adoptive siblings. A conventional summary is that about half of the variation in intelligence, personality and life outcomes is heritable.

“”All traits are heritable” is a bit of an exaggeration, but not by much. Concrete behavioral traits that patently depend on content provided by the home or culture are, of course, not heritable at all; which language you speak, which religion you worship in, which political party you belong to. But behavioral traits that reflect the underlying talents and temperaments are heritable: how proficient with language you are, how religious, how liberal or conservative. General intelligence is heritable, and so are the five major ways in which personality can vary … openness to experience, conscientiousness, extroversion-introversion, antagonism-agreeableness, and neuroticism. And traits that are surprisingly specific turn out to be heritable, too, such as dependence on nicotine or alcohol, number of hours of television watched, and likelihood of divorcing.”

The Second Law concerns the family. The shared environment is that which impinges on both siblings alike. The unique environment is everything else – that which impinges on one sibling but not another.

Studies show that the effect of shared environment is small, just 0 to 10% of the variance.

Adult siblings are equally similar whether they grew up together or apart. Adoptive siblings are no more similar than two people plucked off the streets at random. Identical twins are no more similar than one would expect from the effects of their shared genes.

But note these studies exclude cases of criminal neglect, abuse and abandonment in a bleak orphanage, so they do not show that extreme cases fail to leave scars. Nor can they say anything about the difference between cultures.

The Third Law accounts for the portion not in the first two laws.

In summary – genes 40-50%, shared environment 0-10%, unique environment 50%.

Pinker is convinced that children are socialised in their peer groups and not in their families. But he is not convinced that peer groups explain personalities – shy v bold etc.

Consider identical twins that grow up together. They share all 3 elements but the correlations around them are only 50%. Pinker thinks it is chance events that we haven’t yet identified.

Chance events lead to tiny differences, but these are constrained by human feedback mechanisms used to make sure everything is within tolerance. This chance shaping of brain leading to a difference in personality would be a mixed genetic/environment component.