chapter  7
The Mzt–Dzt Equal Environment Assumption
The Achilles Heel of the Classical Twin Method
Pages 26

This chapter analyses the discussion based on samples of twin pairs reared together in the same family home (MZT versus DZT), which form the basis of twin method investigations. It attempts to show that efforts by twin researchers to uphold the validity of the twin method and its all-important EEA, by using Argument A and/or Argument B, do not hold up to critical examination. The perceived validity of TRA studies is frequently cited in support of the twin method and its underlying assumptions. Researchers in psychology and psychiatry have used the twin method since the 1920s to assess whether genetic factors underlie psychological characteristics such as IQ and personality, major psychiatric disorders such as schizophrenia and bipolar disorder, criminality, and many other areas of human behavior. The investigators chose to emphasize the point that, according to their definition of the EEA, 'the assumption is not that MZs and DZs have equal environmental experiences'.