Social behavioral abnormalities in Zic2kd/+ mice were characterized by a reduction in aggressive behavior compared to the wild-type controls in the absence of clear deficits in the affiliative behaviors. The aggressivity assessed in the resident-intruder and social dominance tube tests may be related to their territory protecting behavior. The absence of depression-like behavior in these mice excludes the possibility that their reduced aggressivity was the result of a general loss in motivation.