Scientists have long known that emotions and other personality traits and disorders run together in families. But finding which genes are most important in controlling emotions has proven difficult. Researchers have noted that male mice make high-frequency vocalizations during sexual interactions with female mice. These high-frequency calls are associated with approach behaviors, and with genes that control positive emotions. In the courtship phase, before mounting, males usually made simple whistles or modulated calls. After mounting the females, however, the males shifted to more complex “chirp-like” vocalizations. The male vocalizations increased in number and complexity as intensity increased.