It’s Time to Radically Reevaluate “Reared-Apart” (Separated) Twin Studies


Summary and Link to the Main Article

It’s Time to Radically Reevaluate “Reared-Apart” (Separated) Twin Studies


By Jay Joseph, Psy.D.

August 9th, 2020

(Link to the full-article pdf HERE)

The “validity of twin studies” question is very important because these studies supply the main “scientific” evidence in support of the “nature” (genetic) side of the long-running “nature-nurture” debate. The question of whether genes or environments are the main cause of human behavioral differences has profound implications for social relations and political policy decisions. 

In a previous article I summarized the critique of “twin method” studies, which use reared-together twins. I argued that the main assumption these studies are based on is false, and I concluded that the twin method should be abandoned as a tool for the detection of genetic influences on behavior. In the present article I summarize and add to the main arguments that I and other critics have made against genetic interpretations of the results of “twins reared apart” (TRA) studies (also known as “separated twin studies”), with a special emphasis on the famous “Minnesota Study of Twins Reared Apart.” Contrary to typical media and textbook descriptions, most twin pairs found in TRA studies were only partially reared apart. In the Minnesota study, I show that the researchers’ strong genetic biases led them to suppress their own control group results in the area of IQ, and that they based their study on numerous questionable or false assumptions. In addition, independent researchers have been prohibited from inspecting the raw data, which includes test scores and information on the degree of separation experienced by twin pairs.

I have developed my analysis of twins reared apart studies in books, peer-reviewed articles, and online articles since 2001After reviewing the numerous problem areas and biases found in these studies, in this most recent review I conclude that no TRA study published to date has provided scientifically acceptable evidence that genetic factors influence human behavioral differences, and that a radical reassessment of behavioral twin research should soon begin.

Length: About 8,000 words, plus notes

Please follow this link to the full article in pdf form: It's Time to Radically Reevaluate "Reared-Apart" (Separated) Twin Studies


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