Two experiments investigating sex differences in the open-field behavior of young chicks are reported. In the first experiment, ambulation latencies of lO-day-old male and female chicks in a novel environment were measured. Half of the chicks were kept in social isolation for 2 days prior to testing; the other half were socially reared until tested. Results showed that in socially reared chicks ambulation latencies were significantly higher in males than in females, whereas in isolation-reared chicks there were no significant sex differences. In the second experiment, latencies of ambulation were measured in socially reared lO-day-old chicks placed in a novel environment with or without a visible predator (i.e., a human being). Sex differences were evident in both conditions, with males showing higher ambulation latencies than females. It is argued that sex differences in open-field behavior of chicks may bedue to a stronger motivation for social reinstatement in females, which reduces the usual antipredatory reactions of chicks placed in a novel environment.Contradictory resultshave been reportedconcerning the existence of sex differences in the open-field behavior of youngchicks. Candland, Nagy, and Conklyn(1963) found no significant sex differences in the distress calling of 1-day-old chicks, and Gallup (1974) reported no sex differences in tonic immobility. Faure and Folmer (1975) and Faure (1979) reported sex differences in only one of 13 strains tested. Consistent sex differences, however, have been reported by Jones and his co-workers: males show less activity, vocalization, and feeding (Jones, 1977a(Jones, , 1977bJones & Black, 1979), and emerge more slowly from a dark enclosure (Jones, 1979). These observations were recently confirmedby Faure (1981,1982), who argued that previous contradictory results could be due to differencesin the open fields used, whichmay have induced differing levels of fear depending on how similar they were to the chicks' home cage. These authors maintainthat (1) males are more fearfulthan females, and (2) sex differences that appear at low-to-moderate levels of fear may disappear at higher levels of fear.What remains unclear, however, is what the functional explanation for these differences may be: why should males be more fearful than females? A possible explanation may be relatedto the differentinterpretative approach of open-field behavior recently proposed by Gallup and Suarez (1980). Gallup and Suarez proposed that, as opposed to a "fear" or "emotionality" hypothesis, openfield behavior might reflect a compromise between opposite tendencies to reinstate social contact with conspecificsand to evade predation. field experiments involve predatory overtones related to the human contact resulting from the handling associated with the removal of the animal from its home cage and its subsequent placement in the testing apparatus (Suarez & Gallup, 1981). Moreover,open-field testing also usually involves suddensocial separation from familiar and/or imprinted companions (Suare...