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Studies have suggested, for example, that people are more likely to select mates with similar levels of intelligence than they are mates with similar levels of neuroticism, extraversion and other personality traits (see page 50).So, researchers who use twins to study intelligence might have to worry more about nonrandom mating than researchers who study personality."And we'll use that DNA to try and identify specific individual genes that contribute to the overall pattern of heritability." APA maintains an archive of our published material throughout our websites.
Others believe it is the environment alone that is responsible for molding humans into who they are.
There is much unknown in this field, but the perusal and review of twin, adoption, and family studies is a significant stepping stone in better understanding this topic.
The value of twin studies Twin researchers acknowledge that these and other limitations exist.
But, they say, the limitations don't negate the usefulness of twin studies.
Twin studies estimate the heritability of a trait, but molecular genetics attempts to pinpoint the effects of a particular gene.
The future of twin research will involve combining traditional twin studies with molecular genetics research, according to Hewitt, who believes that day is already here."When we conduct a study of twins these days, we always get DNA on everyone," Hewitt says.
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: Additive genetic mechanisms, in contrast, mix together--a plant that receives one red gene and one white gene might, if the genes are additive, turn out pink.
She argues that they should not be seen as assumptions at all, but instead as mechanisms whose relevance can be tested using study designs that go beyond the classical twin study design. "People are always thinking about ways to improve the analyses," Hewitt says. "For some designs, we don't have to make as strong assumptions as we used to make," he says.
"Instead of having to assume away four constructs, we only have to assume away two or three."In the age of molecular genetics, meanwhile, the classical twin study design is only one aspect of genetics research.