Stimulus exposure protocols
All protocols were conducted between 09:00-13:00 h. Social stimuli were presented as described for the BL/6 mouse experiment, except on the day prior to the first SR test, a female was introduced into the cage of the male until the first mount attempt, which took 2-15 min, and the female was then removed; this primed the male so that “female” was not a novel stimulus in SR tests (Table S1). In experimental social mice (SR-SA, SA-SR), the order of exposure to social reward (SR) and social aversion (SA) was counterbalanced, i.e. nearly equal numbers of SR-SA and SA-SR mice were studied. In control social mice, the same stimulus type was presented twice, i.e. SR-SR or SA-SA: in the case of SR, if the female that was SR at first exposure was not at (pro-)estrus on the day of second expsoure, then another female was used, and in the case of SA it was always the same CD-1 male in both tests.