I would like to know how to properly account for within-pair variance correlation in FEAT in a twin design. We have measured a continuous variable for all subjects that we are using as a regressor in our fMRI analysis, but we recognize that this variable may not have error that is independent of the twin-pair sampling. In our behavioral analysis in STATA, we can easily implement a design that cluster/nests the twin pairs to obtain robust standard errors. For the neuroimaging analysis, the path is not so clear. I have the following questions:

1) We believe that FLAME 2's full MCMC resampling would automatically produce robust standard errors in this case, but we would like to save time by just running FLAME 1. Does the Metropolis Hastings sampling procedure also result in robust standard errors in this case?

2) At the higher-level, would indicating separate Group membership in FEAT's GLM setup (i.e. Inputs 1&2=Group 1, Inputs 2&3=Group 2, etc) be the same as clustering twin pairs?