Group- and individually-housed mice respond differently to the presence of a novel conspecific, but show no difference in serum corticosterone. Mice were placed in a clean cage and then a novel, female mouse was introduced to the cage for 5 minutes. A) IH mice of both genotypes spent significantly more time in contact with the novel mouse, B) GH mice spent significantly more time rearing up on their hind legs, a behavior associated with investigation of the environment. n = 5 for each group in the novel conspecific test. C) Serum corticosterone (Cort) collected 2-3 hours after the novel conspecific test showed no difference between IH and GH animals. Cort from group- and individually-housed p53+/- animals was compared using a standard EIA kit. GH n = 8, IH n = 8. WT = wild type, p53+/- = p53 heterozygote, GH = group-housed, IH = individually housed. Different uppercase letters denote overall statistical differences in housing or genotype (p≤0.05). Different lowercase letters denote statistical differences found in post-hoc comparisons between experimental groups (p≤0.05). Bars show means ± SEM.