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.
For traits that are substantially influenced by heredity, the approximately two-fold difference in genetic similarity between the two types of twins should outweigh any complications, says John Hewitt, Ph D, director of the Institute for Behavioral Genetics at the University of Colorado at Boulder.
And the extent to which different assumptions matter may depend on which trait is being studied.
Epistatic mechanisms are complex cases where interactions among multiple genes may determine the outcome of one trait.
Twin studies, in general, assume that only one type of genetic mechanism--usually additive--is operating for a particular trait.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.Twin study designs and statistical analysis methods are also constantly evolving and improving.The original twin study design has expanded to include studies of twins' extended families, longitudinal studies and other variations.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.But despite the popularity of twin studies, some psychologists have long questioned assumptions that underlie them--like the supposition that fraternal and identical twins share equal environments or that people choose mates with traits unlike their own.The equal environments assumption, for example, has been debated for at least 40 years."Twins have a special claim upon our attention; it is, that their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives."1875More than a century after Galton's observation, twin studies remain a favorite tool of behavioral geneticists.Researchers have used twin studies to try to disentangle the environmental and genetic backgrounds of a cornucopia of traits, from aggression to intelligence to schizophrenia to alcohol dependence.