Across neuroscience research, the expression "sex differences" has become a catchall term for any phenomenon that we might observe in one sex but not the other-or even a phenomenon observed in both sexes subgroups of males that exhibit distinct patterns of gene expression in the ventral tegmental area (VTA). Intriguingly, these divisions were most tightly predictive of the behavior of the male's female partner, even more so than the behavior of the male himself.Of course, not all social behavior is related to mating, and as three of this issue's research articles demonstrate, genetic modifications can produce sex-dependent effects in a wide range of social outcomes.First, haploinsufficiency of the CACNA1C gene resulted in increased rough-and-tumble play in adolescent female rats, but not males. 6 In contrast, heterozygous males, but not females exhibited altered prosocial ultrasonic vocalizations (USVs). Because CACNA1C is implicated with several mental illnesses that differentially affect men and women (eg, major depressive disorder and schizophrenia), these data may shed light onto a potential mechanism of sex-dependent pathophysiologies. USVs also play a key role in pup-mother communication, and Nolan et al 7 find that male and female Fmr1 (a gene linked to fragile X syndrome) knockout or heterozygous mice exhibit discrete patterns of altered vocalizations compared with their wildtype, same-sex littermates. Finally, in a mutant mouse model of amyotrophic lateral sclerosis (ALS), Kreilaus et al 8 report that prior to the onset of expected motor deficits, mutant males, but not females exhibited reduced sociability, pointing to a potential sex-specific early marker of the disease.Rounding out the issue is a comprehensive review by Brenman-Suttner et al, 9 who explore the complex interactions between genetics, physiology, and time that orchestrate age-related changes in social behaviors. Using D. melanogaster as a model system, the authors discuss a broad range of sex-dependent behaviors, such as courtship, aggression, or egg-laying conditioning, as well as those expressed by both males and females, like social spacing and circadian rhythm entrainment.Together with those in the upcoming companion Special Issue, the articles here comprise a rich sampling of the many ways in which biological sex can contribute to behavioral outcomes in a host of species ranging from fruit flies to humans. The diverse perspectives offered in both issues should serve as food for thought to GB2's readers as they attempt to consider sex as a biological variable in their own research.