In response, though, researchers are working to expand and develop twin study designs and statistical methods.And while the assumptions question remains a stumbling block for some researchers, many agree twin studies will continue to be an important tool--along with emerging genome and molecular research methods (see article page 42)--in shedding light on human behavioral genetics.
The argument of genetic makeup versus environmental influences, however, has researchers working to determine what really shapes us.
Some say genotypes control how people think, feel, and behave.
Even today, the sole sculptor of human personality, behavior, and psychopathology remains unknown; modern research indicates that a combination of biology and environment constructs us all.
Studying how genes and our surroundings may or may not form the personalities, behaviors, and psychopathologies of human beings is probably the most obvious way to approach the argument of heredity versus the environment.
"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.
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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.