New publication: The Twin Research Debate in American Criminology

by Jay Joseph, Claudia Chaufan, Ken Richardson, Doron Shultziner, Roar Fosse, Oliver James, Jonathan Latham, and John Read

Published in Logos (Vol 14) 2015.

Summary: Classical twin research has been one of the most influential research methods in all of biology. Twin research is based on the proposition that human twins can be either monozygotic (genetically identical) or dizygotic (share 50% of their genes) and this genetic difference can be used to infer the magnitude of a putative genetic component contributing to any physical or behavioural trait. Based largely on many thousands of such studies, which usually show that monozygotic twins are significantly more alike than are dizygotic twins, the scientific community at large has concluded that there is a strong genetic component to many human attributes. Characters for which such conclusions have been reached include practically every familiar physical and mental illness (including heart disease, diabetes, Parkinsonism, ADHD, etc.) and also human behaviours such as IQ, voting preferences, and criminality.

The flaw in this logic, which is outlined in this paper, is that this twin methodology makes use of improbable assumptions. Most notable of these is that the environments of monozygotic and dizygotic twins are identical, and in particular that the environments of monozygotic twins are not more alike. This particular assumption is called the equal environment assumption (EEA). This assumption has never been proven. On the contrary, it can clearly be shown to be often false. This casts grave doubt on ALL twin study findings.

The genetic explanations extrapolated from twin studies have almost never been supported by actual positive findings of significant gene variants in human populations (e.g. Manolio et al., 2009). This failure provides another reason to suppose that the twin method is flawed and we propose that this faultiness lies with the equal environment assumption. In other words, the explanation for the higher similarity of monozygotic twins compared to dizygotic ones is not their genes but their more similar environments.

In this paper, we examine the specific flaws in the EEA from the perspective of criminology, but equivalent or identical arguments apply to all twin research. The scientific implication is that most human variation results from environmental variation in physical, chemical, and social factors (or chance) and not from variation between genes or genomes, and that all twin research is effectively worthless. More broadly it also follows that society has been erroneously led by genetic researchers into a genetic determinist mindset that bears little relation to reality.

Manolio T. et al. (2009) Finding the missing heritability of complex diseases. Nature 461: 747-753.