Heylens et al (2012)- found 9 of the MZ pairs were concordant for GD.
However point for Heylens et al's study
Concordance rates are not very high
Hard separate environmental and genetic factors
Small sample.
Supporting evidence
Mitchell (1991)- they found that mothers of boys with GD were more encouraging of feminine behaviour than the mothers in the control group and they were also significantly less encouraging of masculine behaviour.
Contradictory evidence
Chung et al (2002)- claims that pre-natalhormonal influences are not triggered until adulthood. So, although these influences occur before birth the structural changes in the brain do not occur until much later.
Practical applications
Research in this area is socially sensitive as it not seen as abnormal.
However point for Mitchell's study
It is difficult to establish cause and effect – mothers may be reacting to already existing characteristics of their sons, rather than the mothers’ behaviour causing GD.
