The mixed or multilevel model for behavior genetic analysis.

We propose the mixed model or multilevel model as a general alternative approach to existing behavior genetic analysis-an alternative to correlation analysis, the DeFries-Fulker analysis, and structural equation modeling. The mixed or multilevel model handles readily families of behavioral genetic data, which include paired sibling data (e.g., pairs of MZ and DZ twins) and clustered sibling data (e.g., a family of more than two biological siblings) as special cases. Not only can a family of behavioral genetic data have more than two siblings, it can also contain multiple types of siblings (e.g., a pair of MZ twins, a pair of DZ twins, a full sibling, and a half sibling). In contrast to the traditional approaches, the mixed or multilevel model is insensitive to the order of the siblings in a sibling cluster. We apply our approach to a large, nationally representative behavior genetic sample collected recently by the Add Health Study. We demonstrate the approach through several applications using both clustered and family complex behavioral genetic data: conventional variance decomposition analysis, analysis of interactions between genetic and environmental influences, and analysis of the possible genetic basis for friendship selection. We compare results from the mixed or multilevel model, Pearson's correlation analysis, and the structural equation model.