First Partner Choice in a Native Minority: The Role of Own and Parental Ethnolinguistic Affiliation

Despite increasing diversity within many societies, ethnically endogamous unions remain common. In contexts where one ethnic minority has lived alongside the majority for centuries, understanding who partners with whom is central to understanding how ethnic boundaries are maintained or dissolved. This study examines the role of own and parental ethnolinguistic affiliation for the first partner choice in Finland. We provide a unique test of the relevance of ethnic endogamy across two generations, in a context where both groups are native, but one (Finnish speakers) overwhelmingly outnumbers the other (Swedish speakers). Using register data on the total population, we examine how a person's ethnolinguistic affiliation and background affect the choice of the first cohabiting partner in terms of the partner's ethnolinguistic affiliation and background. We apply discrete-time competing risk models for men and women born 1970-1983. Results indicate that Swedish-registered individuals with two Swedish-registered parents are, by far, the most likely to partner with another Swedish-registered person with endogamous background. Partnering with a Swedish-registered person with exogamous background is most likely among individuals who themselves come from mixed unions. Patterns are remarkably consistent across gender, and adjustments for education and residential area only marginally alter the results.

Action planning and execution cues influence economic partner choice

Prudently choosing who to interact with and who to avoid is an important ability to ensure that we benefit from a cooperative interaction. While the role of others' preferences, attributes, and values in partner choice have been established (Rossetti, Hilbe & Hauser, 2022), much less is known about whether the manner in which a potential partner plans and implements a decision provides helpful cues for partner choice. We used a partner choice paradigm in which participants chose who to interact with in the Prisoners' Dilemma. Before choosing a cooperation partner, participants were presented with information about the potential partners' decision-related actions in another round of the Prisoners' Dilemma. They received either information about the potential partners' planning during decision making (i.e., decision-time; Experiment 1) or action execution during decision implementation (i.e., movement directness; Experiment 2). Across both games, participants preferred to interact with those who planned actions quickly or executed actions with direct and smooth movements, indicating that they were cooperating confidently and without deliberation. This demonstrates that action cues present in either the planning or implementation of economic decisions influence partner choice. We discuss implications of this finding for human decision-making and perception-action coupling in action understanding.

Self-interest precludes prosocial juice provisioning in a free choice group experiment in bonobos

Previous studies on prosociality in bonobos have reported contrasting results, which might partly be explained by differences in experimental contexts. In this study, we implement a free choice group experiment in which bonobos can provide fruit juice to their group members at a low cost for themselves. Four out of five bonobos passed a training phase and understood the setup and provisioned fruit juice in a total of 17 dyads. We show that even in this egalitarian group with a shallow hierarchy, the majority of pushing was done by the alpha female, who monopolized the setup and provided most juice to two adult females, her closest social partners. Nonetheless, the bonobos in this study pushed less frequently than the chimpanzees in the original juice-paradigm study, suggesting that bonobos might be less likely than chimpanzees to provide benefits to group members. Moreover, in half of the pushing acts, subjects obtained juice for themselves, suggesting that juice provisioning was partly driven by self-regarding behavior. Our study indicates that a more nuanced view on the prosocial food provisioning nature of bonobos is warranted but based on this case study, we suggest that the observed sex differences in providing food to friends corresponds with the socio-ecological sex difference in cooperative interactions in wild and zoo-housed bonobos.

The Role of Vision in the Emergence of Mate Preferences

Cross-cultural research has repeatedly demonstrated sex differences in the importance of partner characteristics when choosing a mate. Men typically report higher preferences for younger, more physically attractive women, while women typically place more importance on a partner's status and wealth. As the assessment of such partner characteristics often relies on visual cues, this raises the question whether visual experience is necessary for sex-specific mate preferences to develop. To shed more light onto the emergence of sex differences in mate choice, the current study assessed how preferences for attractiveness, resources, and personality factors differ between sighted and blind individuals using an online questionnaire. We further investigate the role of social factors and sensory cue selection in these sex differences. Our sample consisted of 94 sighted and blind participants with different ages of blindness onset: 19 blind/28 sighted males and 19 blind/28 sighted females. Results replicated well-documented findings in the sighted, with men placing more importance on physical attractiveness and women placing more importance on status and resources. However, while physical attractiveness was less important to blind men, blind women considered physical attractiveness as important as sighted women. The importance of a high status and likeable personality was not influenced by sightedness. Blind individuals considered auditory cues more important than visual cues, while sighted males showed the opposite pattern. Further, relationship status and indirect, social influences were related to preferences. Overall, our findings shed light on the availability of visual information for the emergence of sex differences in mate preference.

Nothing better to do? Environment quality and the evolution of cooperation by partner choice

The effects of partner choice have been documented in a large number of biological systems such as sexual markets, interspecific mutualisms, or human cooperation. There are, however, a number of situations in which one would expect this mechanism to play a role, but where no such effect has ever been demonstrated. This is the case in particular in many intraspecific interactions, such as collective hunts, in non-human animals. Here we use individual-based simulations to solve this apparent paradox. We show that the conditions for partner choice to operate are in fact restrictive. They entail that individuals can compare social opportunities and choose the best. The challenge is that social opportunities are often rare because they necessitate the co-occurrence of (i) at least one available partner, and (ii) a resource to exploit together with this partner. This has three consequences. First, partner choice cannot lead to the evolution of cooperation when resources are scarce, which explains that this mechanism could never be observed in many cases of intraspecific cooperation in animals. Second, partner choice can operate when partners constitute in themselves a resource, which is the case in sexual interactions and interspecific mutualisms. Third, partner choice can lead to the evolution of cooperation when individuals live in a rich environment, and/or when they are highly efficient at extracting resources from their environment.

Divorce in Turkish and Moroccan Communities in Belgium

This paper focuses on divorce amongst Turkish and Moroccan Belgians, with a specific focus on the effect of partner-choice patterns. Divorce patterns of marriages established between 01 January 2001 and either 31 December 2003 (descriptive part), or 31 December 2005 (event-history analyses) are analysed and compared to marriages established between 01 January 1988 and 31 December 1990. We distinguish three marriage types: transnational marriages (i.e. marrying a partner from Morocco or Turkey), local intra-ethnic marriages (marrying another Moroccan of Turkish Belgian) and mixed marriages (i.e. marrying someone with a Belgian or other Western-European citizenship). To research divorce rates, we analysed population data from the Belgian national register, using piecewise constant log-rate event-history analyses with effect coding on all marriages taking place between 01 January 2001 and 31 December 2005 (N Turkish = 9631, N Moroccan = 17,786). First, the results reveal that in the past 15 years, divorce rates have doubled within Turkish and Moroccan migrant groups. Second, divorce rates are much higher amongst the Moroccan group. Third, there are clear differences between marriage types. Local intra-ethnic marriages have the lowest divorce levels, mixed marriages the highest, and transnational marriages take up a middle position.

Pay to play: Children's emerging ability to use acts of generosity for selfish ends

Adults will offer favors to advance their standing and solicit a favor in return, using ostensibly prosocial acts strategically for selfish ends. Here we assessed the developmental emergence of such strategic behaviors in which individuals are generous to elicit future reciprocation from others. In a novel experimental paradigm with children aged 3 to 7 years, we tested whether children are willing to share more valuable resources when this act could prompt subsequent reciprocation. In an Experimental condition, children could share a more attractive or less attractive resource with a person who they knew would subsequently choose to play a game with either the children or another individual. In the Control condition, children knew the person would play alone. Across two studies, we found that over repeated trials, 5- and 7-year-olds, but not 3-year-olds, learned to share more valuable resources in the Experimental condition than in the Control condition. This shows that older age groups were able to quickly learn how to influence the subsequent partner choice in a novel situation. We address theoretical questions about the various types of reciprocity as being supported by different psychological mechanisms and discuss whether the current results could be explained by children's emerging ability for future-directed thinking.

Plant-mediated partner discrimination in ectomycorrhizal mutualisms

Although ectomycorrhizal fungi have well-recognized effects on ecological processes ranging from plant community dynamics to carbon cycling rates, it is unclear if plants are able to actively influence the structure of these fungal communities. To address this knowledge gap, we performed two complementary experiments to determine (1) whether ectomycorrhizal plants can discriminate among potential fungal partners, and (2) to what extent the plants might reward better mutualists. In experiment 1, split-root Larix occidentalis seedlings were inoculated with spores from three Suillus species (S. clintonianus, S. grisellus, and S. spectabilis). In experiment 2, we manipulated the symbiotic quality of Suillus brevipes isolates on split-root Pinus muricata seedlings by changing the nitrogen resources available, and used carbon-13 labeling to track host investment in fungi. In experiment 1, we found that hosts can discriminate in multi-species settings. The split-root seedlings inhibited colonization by S. spectabilis whenever another fungus was available, despite similar benefits from all three fungi. In experiment 2, we found that roots and fungi with greater nitrogen supplies received more plant carbon. Our results suggest that plants may be able to regulate this symbiosis at a relatively fine scale, and that this regulation can be integrated across spatially separated portions of a root system.

The costs of being consequentialist: Social inference from instrumental harm and impartial beneficence

Previous work has demonstrated that people are more likely to trust "deontological" agents who reject harming one person to save many others than "consequentialist" agents who endorse such instrumental harms, which could explain the higher prevalence of non-consequentialist moral intuitions. Yet consequentialism involves endorsing not just instrumental harm, but also impartial beneficence, treating the well-being of every individual as equally important. In four studies (total N = 2086), we investigated preferences for consequentialist vs. non-consequentialist social partners endorsing instrumental harm or impartial beneficence and examined how such preferences varied across different types of social relationships. Our results demonstrate robust preferences for non-consequentialist over consequentialist agents in the domain of instrumental harm, and weaker - but still evident - preferences in the domain of impartial beneficence. In the domain of instrumental harm, non-consequentialist agents were consistently viewed as more moral and trustworthy, preferred for a range of social roles, and entrusted with more money in economic exchanges. In the domain of impartial beneficence, preferences for non-consequentialist agents were observed for close interpersonal relationships requiring direct interaction (friend, spouse) but not for more distant roles with little-to-no personal interaction (political leader). Collectively our findings demonstrate that preferences for non-consequentialist agents are sensitive to the different dimensions of consequentialist thinking and the relational context.

The influence of a demographic change on social relationships among male golden snub-nosed monkeys (Rhinopithecus roxellana)

It has been suggested that social relationships are more likely to be prone to variation in the dispersing sex than the philopatric sex. However, we know less about the dynamics of all-male groups in male-dispersing species than we do about other types of primate groups. We studied male sociality in a captive group of golden snub-nosed monkeys (Rhinopithecus roxellana), which was composed of a one-male unit (OMU, N = 7) and an all-male unit (AMU, N = 7 or 8), in Shanghai Wild Animal Park, China. Using data collected for 6 months, during which there was a demographic change in the AMU and the alpha male was replaced by a newcomer, we found that a dramatic change in social ranks occurred accompanied by elevated aggression following this social upheaval. A proximity-based social network analysis revealed that members did not associate randomly any more but formed differentiated relationships post-upheaval, resulting in three distinct sub-units in the AMU. In terms of inter-unit interactions, significant changes were found in the affiliations between the male juvenile of OMU and AMU individuals. He interacted with AMU individuals randomly and frequently pre-upheaval, but cut down his affiliations and had a preferred partner post-upheaval, who was a member of the dominant male's sub-unit. Our findings suggest that social networks in the dispersing sex are dynamic structures and vary by some demographic change (e.g., individual immigration) in the studied species. We also put forward that individual dominance could be a criterion when the male juvenile chooses partners before he immigrates into a group. In conclusion, the high level of behavioral flexibility of the dispersing sex could be an evolutional strategy and good for individuals' future dispersing life.

Fitness costs of symbiont switching using entomopathogenic nematodes as a model

BACKGROUND

Steinernematid nematodes form obligate symbioses with bacteria from the genus Xenorhabdus. Together Steinernema nematodes and their bacterial symbionts successfully infect, kill, utilize, and exit their insect hosts. During this process the nematodes and bacteria disassociate requiring them to re-associate before emerging from the host. This interaction can be complicated when two different nematodes co-infect an insect host.

RESULTS

Non-cognate nematode-bacteria pairings result in reductions for multiple measures of success, including total progeny production and virulence. Additionally, nematode infective juveniles carry fewer bacterial cells when colonized by a non-cognate symbiont. Finally, we show that Steinernema nematodes can distinguish heterospecific and some conspecific non-cognate symbionts in behavioral choice assays.

CONCLUSIONS

Steinernema-Xenorhabdus symbioses are tightly governed by partner recognition and fidelity. Association with non-cognates resulted in decreased fitness, virulence, and bacterial carriage of the nematode-bacterial pairings. Entomopathogenic nematodes and their bacterial symbionts are a useful, tractable, and reliable model for testing hypotheses regarding the evolution, maintenance, persistence, and fate of mutualisms.

Neural bases of the adaptive mechanisms associated with reciprocal partner choice

In our society, partner choice is often reciprocal and, therefore, compromising one's choice may be adaptive depending on one's own market price. To reveal the neural mechanisms underlying this adaptive process, functional magnetic resonance imaging (fMRI) was performed on 27 male subjects during virtual partner choice tasks involving a dance-partner choice or a part-time job choice. Following the evaluation of a rival, the subjects chose a partner either in the face of competition with a rival (reciprocal choice condition) or during no competition (nonreciprocal condition). Irrespective of the type of partner choice situation, the posterior cingulate cortex (PCC) and right temporoparietal junction (TPJ) were specifically activated during reciprocal partner choice. The PCC was also activated during the evaluation of a rival relative to the self, which indicates the involvement of this region in the processing of one's own market price. Activation in the right TPJ was related to the individual tendency to avoid choosing a higher-value candidate when the rival-value was high in the reciprocal choice condition, which indicates that this region plays a role in market-adaptive strategy. Taken together with extant anatomical knowledge, the two-component neurobiological structure underlying the adaptive mechanism of partner choice identified in this study seems to represent the hierarchical evolution of the human socio-cognitive system.

Cooperation and deception in primates

Though competition and cooperation are often considered opposing forces in an arms race driving natural selection, many animals, including humans, cooperate in order to mitigate competition with others. Understanding others' psychological states, such as seeing and knowing, others' goals and intentions, and coordinating actions are all important for complex cooperation-as well as for predicting behavior in order to take advantage of others through tactical deception, a form of competition. We outline evidence of primates' understanding of how others perceive the world, and then consider how the evidence from both deception and cooperation fits this framework to give us a more complete understanding of the evolution of complex social cognition in primates. In experimental food competitions, primates flexibly manipulate group-mates' behavior to tactically deceive them. Deception can infiltrate cooperative interactions, such as when one takes an unfair share of meat after a coordinated hunt. In order to counter competition of this sort, primates maintain cooperation through partner choice, partner control, and third party punishment. Yet humans appear to stand alone in their ability to understand others' beliefs, which allows us not only to deceive others with the explicit intent to create a false belief, but it also allows us to put ourselves in others' shoes to determine when cheaters need to be punished, even if we are not directly disadvantaged by the cheater.

Cooperation in wild Barbary macaques: factors affecting free partner choice

A key aspect of cooperation is partner choice: choosing the best available partner improves the chances of a successful cooperative interaction and decreases the likelihood of being exploited. However, in studies on cooperation subjects are rarely allowed to freely choose their partners. Group-living animals live in a complex social environment where they can choose among several social partners differing in, for example, sex, age, temperament, or dominance status. Our study investigated whether wild Barbary macaques succeed to cooperate using an experimental apparatus, and whether individual and social factors affect their choice of partners and the degree of cooperation. We used the string pulling task that requires two monkeys to manipulate simultaneously a rope in order to receive a food reward. The monkeys were free to interact with the apparatus or not and to choose their partner. The results showed that Barbary macaques are able to pair up with a partner to cooperate using the apparatus. High level of tolerance between monkeys was necessary for the initiation of successful cooperation, while strong social bond positively affected the maintenance of cooperative interactions. Dominance status, sex, age, and temperament of the subjects also affected their choice and performance. These factors thus need to be taken into account in cooperative experiment on animals. Tolerance between social partners is likely to be a prerequisite for the evolution of cooperation.

Ape duos and trios: spontaneous cooperation with free partner choice in chimpanzees

The purpose of the present study was to push the boundaries of cooperation among captive chimpanzees (Pan troglodytes). There has been doubt about the level of cooperation that chimpanzees are able to spontaneously achieve or understand. Would they, without any pre-training or restrictions in partner choice, be able to develop successful joint action? And would they be able to extend cooperation to more than two partners, as they do in nature? Chimpanzees were given a chance to cooperate with multiple partners of their own choosing. All members of the group (N = 11) had simultaneous access to an apparatus that required two (dyadic condition) or three (triadic condition) individuals to pull in a tray baited with food. Without any training, the chimpanzees spontaneously solved the task a total of 3,565 times in both dyadic and triadic combinations. Their success rate and efficiency increased over time, whereas the amount of pulling in the absence of a partner decreased, demonstrating that they had learned the task contingencies. They preferentially approached the apparatus when kin or nonkin of similar rank were present, showing a preference for socially tolerant partners. The forced partner combinations typical of cooperation experiments cannot reveal these abilities, which demonstrate that in the midst of a complex social environment, chimpanzees spontaneously initiate and maintain a high level of cooperative behavior.