It’s great to criticise studies and paradigms (more criticism is needed in science!) but if you’re keen to stimulate a true discussion (with no presumptions on what’s right and wrong – after all this is science, there is no simple right and wrong) then I suggest more evidence-base and proportion (on both sides of the argument). It’s sad to read such an undifferentiated and dogmatic article, especially by someone who has been working in academia and should be aware that science is no black-or-white business. Of course being differentiated doesn’t sell as well. If a method has flaws (which I would argue is the case for all methods ever used in science, even in the supposedly “exact” sciences!), that doesn’t mean it has to be abandoned completely. Flaws should be attended to; where possible, assessed and an effort should be made to improve the method. If a whole method would be ditched because it has flaws, there simply wouldn’t be any science. I doubt that’s what you’re after. A contribution to the discussion: “The equal environment assumption across zygosities assumes that environmentally caused similarity is roughly the same for both types of twin pairs reared in the same family. This assumption is the most basic assumption of the twin method and has been the subject of great debate over the years. It is generally agreed that MZ and DZ twins do share their environment to the same extent in many respects: they share the womb at the same time, are exposed to the same environmental factors, are raised in the same family and are the same age. However, there is also some evidence that MZ twins are treated more similarly by their parents and have more frequent contact as adults than DZ twin pairs.(13) • Implications: more similar treatment of MZ twins will increase their correlations relative to DZ correlations, which can result in an overestimation of the genetic effect and an underestimation of the shared environmental effect. (Note: there are also factors that can have the opposite effect and increase variability between MZ twins. One example is when MZ twin pairs are forced to attend different classes at school, while DZ twins are allowed to remain in the same class. This could lead to an underestimation of the genetic effect.) • How do we detect this effect?If parental treatment is more similar for MZ twins, than DZ twins who are mislabelled as MZ twins should be more alike than correctly labelled DZ twins and conversely, MZ twins mislabelled as DZ should be less alike than correctly labelled MZ twins. Little or no effect of mislabelling was found. The effect of degree of contact among twins showed that more frequent contact does not lead to behavioural similarity in same-sex DZ or MZ twins. While in some cases MZ twins in frequent contact were more similar than those with less contact, these correlations tended to be small (16). Another argument in defence of the equal environments assumption is the fact that studies of MZ twins reared apart have provided correlations for personality variables that are almost the same as those for MZ twins reared together (17) (13) Plomin R., DeFries, J. C., McClearn, G. E. and McGufﬁn, P. (2001), ‘Behavioral Genetics’, 4th edn, Worth Publishers, New York (16): Kendler, K. S; Heath, A., Martin, N. G. and Eaves, L.J. (1986), ‘Symptoms of anxiety and depression in a volunteer twin population: The etiologic role of genetic and environmental factors’,Arch. Gen. Psych., Vol. 43, pp. 213–221. (17) Bouchard, T. J. and McGue, M. (1990), ‘Sources of human psychological differences: the Minnesota study of twins reared apart’, Science, Vol. 268, pp. 223–228 All taken from “Analytic approaches to twin data using structural equation models” by Rijsdijk et al. (2002).