First—let me provide some examples of "personality traits" that have been studied in the context of nature-nurture studies. Shaffer Chapter 4 describes the results from one type of personality test (the CPI). Other important personality tests have also been used: Big 5 personality traits: Extraversion-Introversion; Neuroticism-Emotional Stability; Conscientiousness; Agreeableness; Openness to New Experiences. Also, IQ; interests and talents; Psychophysiological Responses (EEG, heart rate, blood pressure); social attitudes have also been studied).

With few exceptions, the conclusions from studies of these variables are largely the same—differences between individuals are due in large part to differences in genetics—a concept known as HERITABILITY—degree to which individual differences are due to genetics. IT’S ABOUT THE SOURCE OF DIFFERENCES—NOT ABOUT THE EFFECTS OF GENES VS. ENVIRONMENT ON THE TRAIT. Calculated based on comparisons of MZ and DZ twins, MZA and MZT, unrelated household members. CONCORDANCE…different from HERITABILITY

Findings that contribute to the conclusion of HERITABILITY of personality traits:

· MZ twins (reared apart or together) are far more similar than DZ twins

· MZA are no less similar than MZT

· Non-related members of same household no more similar than random individuals

o Rearing environment contributes little to individual differences

§ Conclusion limited to normal range of environments—not included are extremes in environmental experience

· BUT—MZ twins are not 100% concordant on any behavioral trait measured. Any differences between MZ twins are due to unique experiences to each individualànon-shared environmental influences…more about this later…

· For most personality traits, there appears to be around 50% heritability—about half of the variability among individuals in personality is due to genes—

-When MZT=MZA, heritability is high

-When MZT>DZT, h is high (but we cannot rule out similarities due to shared environments, because MZ may be treated more similarly than DZ)

-When MZT>MZA, heritability is low (use accent/dialect as example)

-When biological siblings > adopted siblings, heritability is high

When all these techniques converge on the same answer, then the heritability estimates are to be believed.

Remember our caveats: we cannot comment on effects of environments that are outside the range tested—all of the samples studied are normal, middle class households. However, that is a huge range, and the range of normal personality types has been observed in these samples. We need variability in order to do behavior genetics research—without variability in traits, we would have no way of attributing variability either to genes or environment.

Using these techniques, you can identify both the heritability and the effects of the shared environment.

-subtracting heritability from concordance (measure of similarity) among identical twins reared together

-or concordance rate among adopted siblings (even virtual twins)

This tends to get confusing so I want to make sure you understand what this refers to

(diagram drawn on board)

For most personality traits studied, amount of variability that can be attributed to heritability roughly 40-50%, differences between individuals due to being raised in different environments is roughly 0-10%, differences between individuals due to unique, outside the home experiences (likely outside the home, because otherwise, they’d make siblings more alike, which they’re not…no more alike then we’d expect them to be because they share genes) during their lifetimes

THIS IS WHAT MAKES MZA NOT IDENTICAL IN EVERY RESPECT

Here is where the notion of unique environment—non-shard experiences that one member of the twin pair has, the other does not.

Pinker discusses the theory that these unique outside-the-home non-shared experiences are responsible for the variations in our personalities but he also disagrees with the assessment in one important respect. He feels that although these peer group experiences can determine aspects of our behavior…our socialization—how we determine what is the appropriate behavior in a given set of circumstances—he feels that the peer group is unable to influence stable aspects of personality. So if differences in personality are due partly to genetics and only marginally to the upbringing that you share with your siblings, then where does the rest of the variation come from? And here is where he invokes chance—random effects of the environment in shaping who we are.