Two group analysis (MZ and DZ): no sex effects on variances, with or without sex effects on means
In a two zygosity group analyses (including an MZ and DZ datagroup) no sex effects on variance components are estimated. In the two-group scripts, however, we do allow estimation of sex effects on the means, assuming that in fact information on sex is available, and therefore also information allowing a 5 or 6 zygosity group analysis.
This may seem confusing: yet we advise to use the 5 or 6 groups scripts before using the two-group scripts. Only after having established the absence of significant differences across sexes in variance components, one may use the simpler two groups scripts.
Since an effect of sex on the means is independent from an effect of sex on variance components, one still has the option of including an effect of sex on the means in the two zygosity group scripts.