Deficiency of normal estrogen receptor (ER) gene function led to behavioral change in female mice (ERKO females). Maternal behavior as measured by retrieving of pups was reduced. In some cases, pups were killed by the ERKO females, which was not seen in wild-type animals. Aggression toward other females was increased. Female-typical lordosis behavior was reduced for at least two reasons: less response to somatosensory stimuli on the hindquarters, as well as the fact that ERKO females were immediately treated as intruder males by resident ‘stud’ males and were thus attacked. In sum, disruption of the ER gene led to a pattern of hormonal and neural changes which caused the females to lose their normal female-typical behavior and to behave and be treated more like males.