When someone starts playing, others are more inclined to join in. Social hominids such as children, chimpanzees, bonobos, and gorillas exhibit this behavior. A study from the University of Turin, published in the journal Philosophical Transactions of the Royal Society B, demonstrates this. The study examined the "contagion" nature of play among the four species. The study documents this phenomenon for the first time in bonobos and gorillas and identifies several common factors associated with its spread, from individual characteristics to social dynamics within the group. The study also addresses the origins of so-called collective behaviors, such as how play spreads within a group and what factors increase the likelihood of individuals joining. Playing provides a setting in which cooperation and competition can be explored without escalating into conflict. Understanding whether and how this behavior spreads among our close ancestors (other social hominids) can aid in reconstructing its role in the evolution of sociality. Between February 2022 and February 2025, the research team logged approximately 1,100 hours of observation on 107 people, divided into eight groups of two per species. Human data collection took place at Turin's Armando Melis nursery school, with 28 children aged 13 to 36 months. Chimpanzees, bonobos, and gorillas were observed at several European zoos. This research was entirely observational, and it was conducted without interfering with the natural and spontaneous interactions between individuals. To separate contagion from pure coincidence, the researchers looked at what happened in the minutes after the beginning of a play episode. They concentrated on those around who were not yet playing: would they start a new game, either alone or with others, or would they join in on an existing one? The behavior of the same individuals was then compared to that recorded under similar settings but without a preceding play episode. Compared to the no-play condition, the probability of initiating play was substantially higher in all four species after someone else had begun playing. One of the most intriguing findings is the dynamics of social interactions among group members. Contagion was more prevalent in groups where friendly contacts were more common than confrontational ones. This conclusion implies that in order to understand how play spreads, we must also consider the group's network of relationships. Younger individuals were more likely to respond to others’ play, as were individuals who were closer to the person who initiated the activity. Contagion was also more common in groups with a larger proportion of males to females.