In a rare, decades-long study, researchers have documented what appears to be the first observed “civil war” in wild chimpanzees. The findings demonstrate that shifting social ties alone can fracture a once unified group and ignite sustained, deadly conflict among former allies. In humans, war and collective violence are often explained by cultural differences that bind groups while fueling hostility towards outsiders. However, this view cannot fully account for conflicts that arise in once-unified communities, as seen in violent rebellions or civil wars. An alternative explanation argues that shifting social relationships and local rivalries alone can fracture groups and produce violence. Despite lacking human cultural systems, chimpanzees – one of our two closest living relatives – exhibit forms of organized aggression and lethal violence against those who were once group companions. However, direct observations of these events in wild populations have remained elusive.

Drawing on 30 years of behavioral observations and demographic data, Aaron Sandel and colleagues describe a rare and well-observed permanent split and subsequent lethal conflict within the Ngogo chimpanzee group of Kibale National Park, Uganda. It’s estimated that such events occur only once every 500 years. According to Sandel et al., beginning around 2015, the chimpanzee community began to rapidly split from a single cohesive group into two distinct polarized clusters – a social rupture that was matched by spatial and reproductive separation. By 2018, the split was complete and enduring, with no remaining ties between the two groups. As this division solidified, aggression between the two groups escalated. Following the 2018 split, one of the chimpanzee groups launched sustained and coordinated attacks on the other, marking a clear shift to lethal conflict among former group members. These raids resulted in multiple killings of adult males and, beginning in 2021, expanded to frequent infanticide, averaging several deaths per year. The authors note that the true toll of this violence is likely higher than observed, as many individuals disappeared without clear cause, suggesting additional unrecorded attacks. Chimpanzees who had long cooperated and bonded turned on one another after the split, indicating that group identity can be redefined beyond mere familiarity. Sandel et al. suggest that factors such as unusually large group size, competition over food and reproduction, deaths of key individuals, leadership changes, and disease may have destabilized social ties and contributed to the division. “A hostile split among wild chimpanzees is a reminder of the danger that group divisions can present to human societies,” writes James Brooks in a related Perspective. “The study of Sandel et al. also reinforces the importance of maintaining long-term field research sites and of preserving endangered species. Many valuable scientific insights have only been possible because of the commitment and sustained cooperation of those who study and support these species in the wild.”

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Punishing freeloaders in public good games (PPGs) – experimental models used to analyze the social dilemma between individual self-interest and group cooperation – can boost cooperation, but whether punishment helps or harms the groups’ outcomes depends heavily on context, according to a study involving over 7,000 human participants. The findings reveal when, rather than whether, punishment works. Human societies routinely confront so-called “social dilemmas” – situations in which individual incentives clash with the collective good. These scenarios can arise in contexts ranging from public health to environmental policy. When they occur, cooperation – or prioritizing shared benefit over personal gain – is both essential and difficult to sustain. PPGs offer a simplified model for examining these dynamics. Although the group benefits most when everyone contributes fully, individuals can maximize their personal gain by contributing nothing. One widely studied solution to this problem is costly peer punishment, where individuals penalize those who fail to contribute. While this mechanism can discourage selfish behavior, it comes at a cost to both punisher and punished. Past studies have shown that, in some cases, the burden of punishment outweigh its benefits. However, despite this body of research, the conditions under which punishment best promotes cooperation remains unclear.

To better understand these dynamics, Mohammed Alsobay and colleagues conducted a large integrative experiment, systematically varying 14 features of PPGs (e.g. communication, group structure, incentives) across 360 conditions, analyzing more than 147,000 decisions from 7,100 participants. According to Alsobay et al., the experimental design allowed them to precisely identify when punishment helps or hinder shared outcomes, which factors matter most, and how they interact. The authors found that punishment consistently increased cooperation, but its effect on collective welfare varied dramatically – from a 43% improvement to a 44% reduction – depending on context. According to the study, communication was the most influential factor and was roughly three times more consequential than any other variable. Other important elements include how contributions are framed, the structure of contribution choices, the duration of the interaction, and the visibility of others’ outcomes. Notably, the findings show that these factors do not operate in isolation but interact in complex ways. For example, longer interactions only enhance the effectiveness of punishment when communication is possible. The authors also used the data to develop and train a predictive model that was able to outperform humans when predicting whether punishment would help or harm welfare in new experiments.

As global population aging advances and countries face shrinking workforces, a new study focusing on China by IIASA researchers and colleagues from Nanjing University reveals how economic growth can persist despite these changes in age structures.

The aim of this conference is to discuss, from an interdisciplinary, sociological, and global perspective, how cultural diversity is represented in funded literary translations—both past and present—and how these shape the global circulation of texts, the publishing market, and cultural policies.