Volunteer bias in twin research: The rule of two‐thirds

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Abstract
Studies of adult same‐sex twins which rely upon volunteer subjects typically consist of about two‐thirds female and two‐thirds monozygotic (MZ) pairs. Because of this recruitment bias, the male and dizygotic (DZ) twins in such studies will be less representative of their respective populations and will show smaller between‐pair variance on many traits than will the comparison samples of female and MZ twins. This reduction in between‐pair variance results in underestimation of the true intraclass correlation in the populations of DZ twins and in overestimation of the true heritability of the trait under study. The only adequate solution to this problem may be to provide sufficient extrinsic incentive (e.g., money payments) to overcome the strong recruitment bias documented in this paper.