The first law of behavioral genetics is that everything is heritable. By that, I mean individual differences in traits of cognitive abilities, personality, and mental health & illness. Those traits—those individual differences—all show significant and substantial genetic influence.

Adopted children don't correlate with their adoptive parents in terms of body mass index—even though they share food and a similar lifestyle. Parents who share genes and an environment with their children correlate by ~.3 for body mass index. The real killer data is that adopted children correlate by .3 for body mass index with their birth parents who they never saw after the first week of life.

Genetic influences on complex traits and common disorders aren't due to 1 gene, 10 genes, or even 100 genes; they're probably due to thousands of tiny, tiny DNA differences.

Your DNA isn't all that matters, but it matters more than anything else, and it matters more than everything else put together in determining who we are.

If one of your daughters had been switched at birth in the maternity ward and raised in a different family, she would have grown up to be very similar to who she is even though she was raised in a different family.

Many traits—in fact, I would say all traits in psychology—show significant genetic influence.

About 70% of the variance in body mass index among northern European populations that we study is due to inherited DNA differences.

Heritability is a descriptive statistic, and like all descriptive statistics, such as mean and variance, it can change in populations over time. It describes the extent to which differences that we observe in a trait, like body weight or body mass index—to what extent are those differences due to inherited DNA differences between people in this population at this time?

On average, across all traits that have been studied, about half of the differences between people (half of the variance of these traits), can be ascribed to inherited DNA differences.