To functionally test the hypothesis that the observed co-morbidity between T2D and psychiatric disorders might be partially due to pleiotropic effects of TCF7L2 alleles, we performed several behavioral studies using mice with null alleles of Tcf7l2 as well as mice over-expressing Tcf7l2. The use of these mouse models allowed for the testing of a range of Tcf7l2 copy numbers that might be similar to gain- or loss-of-function alleles in humans. Here, TCF7L2 is linked to psychiatric disorder.