The classical twin study design relies on studying twins raised in the same family environments. Monozygotic (identical) twins share all of their genes, while dizygotic (fraternal) twins share only about 50 percent of them. So, if a researcher compares the similarity between sets of identical twins to the similarity between sets of fraternal twins for a particular trait, then any excess likeness between the identical twins should be due to genes rather than environment.Researchers use this method, and variations on it, to estimate the heritability of traits: The percentage of variance in a population due to genes. Modern twin studies also try to quantify the effect of a person's shared environment (family) and unique environment (the individual events that shape a life) on a trait.The assumptions those studies rest on--questioned by some psychologists, including, in recent work, Jaccard--include:

A minority of people, however, cannot get enough social interaction, and some outright dread it. 

Using a newly applied scientific technique, researchers at the Keck School of Medicine of USC have reached surprising findings about the role of nature versus nurture in the development of the neural circuits in the auditory cortex, the area of the brain responsible for processing information about sound.

I am a neuroscientist and I am the father of two-year old triplets—two identical and one fraternal. As a professor at Yale University, I spend most of my time designing experiments, researching or teaching about the brain and the nervous system. The rest of my time I spend surrounded by my three daughters. To understand the dynamics in my household, think terrible-two’s, and then cube it.