For a surprising number of twins, beliefs about whether they were MZ or DZ were incorrect.
When the two did not coincide, genetic tests of MZ were a better predictor of similarity in IQ than twins' beliefs about whether they were identical
This finding shows that the argument about more similar environments for MZ (MZ twins treated more similarly because they are thought to be more alike, and this more similar environment affecting IQ) can be only partly valid
A further problem (which partly invalidates hereditarian arguments), particularly in the US, is that it is very hard to eliminate the possibility of environmental contributions to between-group differences
For example, it might be thought that equating blacks and whites for socio-economic status would control for the generally poorer environment of blacks
However, this ignores the fact that even high SES black may experience discrimination in a way that whites do not