Hi, I'm very new to openMX and I'm trying to analyse some data using ACE models. I have several data (psychological scales) about twins, and I want to calculate cross-twin and within-twin correlations between these scales. I have found such calculations are very common in papers such as this one:

http://www.ncbi.nlm.nih.gov/pubmed/21646580

and they all mention something like this:

"To estimate MZ and DZ twin correlations within and across

each of the 5 OC symptom dimensions, the data were fitted to a

constrained correlational model in Mx. The MZ and DZ correlations

for each of the symptom subscales (ie, the cross-twin withinsubscale

correlation) provided an indication of the relative contribution

of genetic and environmental factors to the liability of

each dimension (eg, checking)."

I understand this I can't just calculate the regular correlation, since the pairs of twins are inter-changeable within them, so this is more like an intra-class correlation.

I've been checking a lot of openMX documentation, but I can't find how to exactly do it. I understand I need to add constrains to the model, but I don't know how.

Any example or guiding light will be very appreciated.

Afterwards I'll try to fit an ACE model to the data. The univariate example found in the openMX documentation is very clear so I hope I won't have trouble, but I might have more trouble when I try a bivariate one.

Thanks a lot in advance!