What the replication of experiments does and doesn’t achieve

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– by Stuart West & Max Burton-Chellew

Replication or repeating of experiments is a key part of the scientific methodology. It increases your trust in that result. It shows that the result was not just due to some unnoticed bias or a particular setup. This helps to filter out false positives and expose questionable research practices. That is great.

Suppose, however, that you had developed a hypothesis for a result. In that case, repeating the same experiment might not increase your confidence in that hypothesis. If your aim is hypothesis testing then it can be more useful to alter the experiment, so that you can test between competing hypotheses or make a strong attempt to falsify a hypothesis.

Our paper was about what happens if these two points get confused. This could happen anywhere, but we illustrate the potential problem with the literature on public goods games.

The public goods game is an economic experiment designed to investigate the tension between an individual’s selfish interest and the interest of the group. Individuals can contribute money to a group fund. The group fund is multiplied by a factor and then divided between players. Crucially, the amount it is multiplied means that everyone gets back less from their contribution than they initially contributed. Consequently, while everyone would do best if everyone contributed maximally, individuals maximise their personal income by not contributing.

But when people play the public goods game, they do contribute. On average, individuals contribute a significant amount. If they play the game multiple times they contribute less, but they keep contributing. This is an incredibly strong result – the experiment has been repeated >100 times. The common conclusion is that because individuals contribute >0%, this shows that humans have ‘prosocial preferences’ which make them cooperate even when it is not in their best interest.

Can you see a potential problem with this conclusion? Individuals would only appear to behave as if they are trying to maximise their personal gain by contributing nothing (0%). Any other amount (>0%) can be argued to reflect pro-social preferences. And it is possible to come up with a huge number of alternative hypotheses for why individuals might contribute >0% (see figure above). ‘Pro-social’ preferences is a hypothesis not an unavoidable conclusion. Alternative hypotheses include, but are not limited to:

  • Individuals are prone to playing as if it was the real world, where there would be repeated interactions, and hence the opportunity for reciprocal helping.
  • Individuals might consider extreme strategies like 0 or 100% as risky.
  • Individuals might start a bit confused and want to use the game to learn how to best maximise their gain.
  • Individuals might want to explore the options available.

So, while repeating the basic public goods game experiment was great for proving the robustness of the initial result, it doesn’t help distinguish between competing hypotheses. To do that, you need to adjust your experiment. You could play the game in a way that removes the potential concern for others. For example, by making individuals play with computers or by not letting them know about the game they are playing (contributions are to a ‘black box’). Or you could change the game so that the best selfish option is to contribute >0%. Or could analyse individual behaviour and see what makes them alter their contribition. And so on.

Another way of thinking about this issue is that we need to get to the right answer by as rigorous a scientific method as possible. This can involve actively developing different controls and null hypotheses. We need to work hard to try to falsify hypotheses and test competing hypotheses.

That is our main point, and you can stop here. If you want to know the gory details from public goods games, how different controls can be developed, and how to test competing hypotheses then go and read our paper. You might also be wondering what would happen if this approach overturned the accepted conclusions. No spoilers, but there is an obvious comparison to a folktale by Hans Christian Anderson: The Emperor’s New Clothes.

Stuart A. West & Maxwell N. Burton-Chellew. (2026). Replication of experiments and the canonisation of incorrect conclusions. Evolution & Human Behavior, 46, 106749.

Evolution in the Public Eye Award Nominations (Feb 7 deadline)

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HBES Members,

Help recognize people who have gone above and beyond in communicating and countering misinformation about human evolutionary science by nominating them for the 2026 Evolution in the Public Eye award! Self-nominations allowed too.

Submission deadline extended until February 7th at: https://forms.gle/VXJgczHA1irDDNB19

Last year’s winner was our president, Ed Hagen.

HBES 2026: Registration is now open (and on-campus accommodation requests are live)

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Dear HBES colleagues,

Below find important information for the HBES conference:

Registration: The registration for the 2026 meeting in Rabat, Morocco, is now open! Please visit the website for information regarding registration, or directly access the registration form. The early bird period will conclude on the 29th of March 2026. Online attendance is possible. If you wish to attend but not present at the CES conference, you can do so at a fixed discounted fee (use the HBES registration form to do so). You can also buy up to 2 tickets for the conference gala dinner, which will take place at the beautiful Rabat Story Hotel – Le Carousel (shuttle from campus included) on the evening of May 16th. Also available are two excursions to Roman ruins–the Chellah in Rabat and the Volubilis ruins closer to Meknès (both UNESCO World Heritage Sites)–planned for May 14th, to which you can also register (transportation and guide included).

Accommodation: The conference is hosted on the Rabat (Salé) campus of the UM6P. The campus offers a range of accommodation options, including the campus hotel and student residences. Prices vary based on the chosen option. Student residences are highly recommended for student participants as they are very cheap. Both are within walking distance of the event venue (approximately 3 minutes). Please note that children, while admitted on campus, are not allowed to reside in the on-campus accommodations. You can book your on-campus accommodation with this form (subject to availability).

Off-campus accommodations are available (see the website for suggestions). A regular shuttle service will pick participants near the Onomo Hotel downtown Rabat (in the Hassan neighbourhood: location here). The stop is also close to a tramline linking with hotels located in Agdal (such as the Marriott or First Suites hotel; about 10 minutes). An additional shuttle service may be provided based on the preferred location of the participants. Please note that the shuttle service is located about 20-30-minute walk uphill if you choose to reside in the Old Medina.

We are also issuing an additional call for Special Activities (e.g., workshops, training sessions, roundtables, networking events, interest-group meetups, and other community-building programming). If you would like to propose a Special Activity, please review the guidelines below and submit a proposal by email (to: HBES2026@um6p.ma). Submissions are due March 15th.

Childcare services: A free childcare center will be organised for the conference, for children from 11 months to 6 years of age. To book a space contact the organizers directly at HBES2026@um6p.ma by March 16th 2026. A discreet changing and breastfeeding space will also be provided (no booking required).

Call for activities proposals: You are invited to submit proposals for two activities to be held during the conference. All activity proposals and further questions should be sent to HBES2026@um6p.ma More details on Special Activities (including formats, expectations, and submission instructions) are included below and available at this link.

Warmly,

Zach Garfield, Ed Seabright, Sarah Alami, Mathieu Charbonneau, HBES 2026 Organizing Committee
Conference website

HBES2026 – Call for activity proposals

You are invited to submit proposals for two activities to be held during the upcoming HBES 2026 conference:

1.⁠ ⁠Lunchtime Panel Discussion

We invite proposals for a panel discussion (about 90 minutes) to be held during lunch, tentatively on the final day of the conference (May 16th). This panel should address an important topic for the discipline that is not strictly scientific but highly relevant, such as issues of representation and inclusion, ethics or equity in the field, history of the field, and broader issues shaping disciplinary culture. Proposals will be reviewed, and only two panel submissions will be selected for inclusion in the program.

Deadline: March 15th, 2026

2.⁠ ⁠Lunch Across Societies – Joint with CES

Held on the joint/overlapping day with CES (May 13th), Lunch Across Societiesoffers an informal space for small-group scholarly exchange aimed at fostering cross-societal dialogue and intellectual exchange in an informal setting. We welcome proposals for brown-bag–style interactive sessions (about 90 minutes), including methods clinics, special-interest or thematic discussions, discussions aimed at supporting junior researchers in their networking and professional development, or any other community-building discussions. Proposals will be reviewed, and a limited number of activities will be selected.

Deadline: March 15th, 2026

Submission Guidelines for 1 and 2 

Please include a short description (max 250 words) of your proposed activity or panel, outlining the following:

  • ⁠Format and topic
  • ⁠Intended audience and relevance
  • ⁠Any logistical needs

All activity proposals, video submissions, and further questions should be sent to HBES2026@um6p.ma. Please include your name and HBES2026 – Call for activities subject line.

Whom Should We Help? People Track Shared Fate to Solve Cooperation Dilemmas

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– by Diego Guevara Beltran

English

Imagine you live in a small community where food is often scarce; storms, droughts, pests, and fires can destroy crops, disease and injuries can keep you from working, and help is often the difference between getting by and going hungry. Every day, you face decisions about cooperation: Whom should I help? How much should I give? And who will help me when I need it? These are not abstract moral questions. They are practical problems that humans have faced throughout evolutionary history. And, they raise a fundamental question about evolution and human behavior: how do people decide whom to help when cooperation is costly and resources are limited? Two long-standing answers are kinship and reciprocity. But these explanations leave some important things unresolved. People often cooperate with non-kin, even when reciprocity is uncertain. Not only that, but in natural fertility populations, people often have more available kin (e.g., cousins, siblings, in-laws) than they could realistically afford to cooperate with. So, what psychological mechanisms allow people to flexibly navigate these partner-choice dilemmas? Drawing from biological markets and fitness interdependence theory, my colleagues and I argue that a key part of the answer lies in how people perceive shared fate.

Shared fate is the extent to which people believe that another person’s outcomes—good or bad—will affect their own. If your sibling falls ill, that may directly affect your workload, your food security, your coalitional strength, or your childcare needs. If your fishing partner succeeds, you may eat better too. In contrast, if an acquaintance struggles, their misfortune may have little consequence for you. From an evolutionary perspective, shared fate is a psychological estimate of fitness interdependence—the degree to which two individuals’ survival and reproduction are yoked. The central idea is this: people should be more willing to help those with whom they share positive fitness stake. But where do these perceptions come from, and what cues do people use to infer shared fate?

To address this question, I conducted fieldwork among the Mayangna, an indigenous small-scale society in northern Nicaragua. The Mayangna rely primarily on horticulture, subsidized by fishing, domestic animals, limited hunting, and sparse or seasonal wage labor. For the Mayangna, as is the case for many subsistence societies, recurrent risks such as food scarcity, illness, and disasters are part of everyday life. Absent wage-labor security or institutional support, cooperation is often the best available insurance against life’s many challenges. We interviewed 146 adults and asked them about their relationships with three types of people: an acquaintance, a cousin, and a sibling. For each relationship, we measured: (1) Ten sources of interdependence, such as relatedness, childhood co-residence, eating together, farming, hunting or fishing, labor sharing (i.e., household construction), shared religion, and experiencing disasters (i.e., floods/hurricanes) together; (2) Perceived shared fate, measured with items like “What is good for [target] is good for me” and “When [target] succeeds, I feel good”; and (3) Cooperation across seven fitness-relevant domains, including giving meat or fish, sharing crops, cooking or preparing meals, helping children, helping with household construction tasks, assisting after disasters, and helping someone harmed by outsiders.

We then tried to address a straightforward question: which sources of interdependence shape perceptions of shared fate, and do those perceptions guide cooperation? As you might suspect, not all interdependence cues are made equal. Most sources of interdependence were correlated with higher shared fate between participants. But, when we examined which cues were most diagnostic across relationships within participants, three stood out: relatedness, commensality (regularly eating together), and shared subsistence activities (i.e., planting/harvesting, and hunting or fishing). These are not arbitrary cues. They are attributes or activities that directly tie people’s outcomes together. Kinship links long-term fitness interests; while eating, farming, and foraging together not only allow people to pool risks, but such activities may also communicate to partners that we value their welfare above others. Importantly, other plausible cues—such as childhood co-residence or sharing the same religion—were not associated with shared fate once the core sources were accounted for. This suggests that shared fate is not simply about tracking social closeness or group affiliation per se. Shared fate is about tracking fitness-relevant cues of interdependence.

Perceived shared fate, in turn, was strongly associated with cooperation. People were more likely to help those with whom they perceived higher shared fate across every domain we measured—from sharing food to helping after floods. What’s more, shared fate statistically mediated the association between relatedness and helping. This suggests that shared fate is a key psychological mechanism by which kinship structures much of the cooperation observed within Mayangna communities—and perhaps across other human societies too. Finally, we wanted to test whether shared fate was associated with actual costly behavior, not just self-reports. We ran a follow-up study with a subset of participants, and we gave them a real choice: keep money for themselves or use it to buy rice for a specific partner. Shared fate was strongly associated with choosing to give up money to help someone else.

Although one interpretation of this work is that our findings simply reaffirm the importance of kinship for cooperation, we argue that such conclusion would be too narrow. The broader implication is that humans appear to possess a flexible psychological system for estimating cues of fitness interdependence, one that integrates multiple cues according to ecologically relevant affordances. In societies where cooperation is largely structured by kinship, as is the case for the Mayangna, shared fate should track kinship closely. In societies where fitness depends on cooperation with non-kin—such as friends, neighbors, or labor/exchange partners—shared fate should track such corresponding cues, including a history of shared labor, risk pooling, or even cooperation itself.

In sum, perceived shared fate may offer a simple solution to partner choice dilemmas: help those whose welfare is linked to yours. Following this heuristic allows individuals to mitigate opportunity costs, protect themselves from the costs of defection, and reap the rewards of cooperation. Understanding how people come to perceive shared fate with others could help us understand not only cooperation in small-scale societies, but also friendship, social support, and collective action in response to larger scale disasters. When people feel that their futures are intertwined, cooperation follows. When that sense of shared fate erodes, so too does willingness to help. Understanding shared fate could be as relevant for understanding how people in modern nations deal with the shadow of disasters (e.g., climate change, intergroup conflict) as it is for small-scale societies past and present.

Spanish

¿A quién deberíamos ayudar? Las personas monitorean el destino compartido para resolver dilemas de cooperación

Imagina que vives en una comunidad donde los alimentos suelen ser escasos; tormentas, sequías, plagas e incendios pueden destruir las cosechas; las enfermedades y las lesiones pueden impedir que trabajes; y la ayuda suele marcar la diferencia entre salir adelante o pasar hambre. Todos los días enfrentas decisiones sobre la cooperación: ¿a quién debería ayudar? ¿Cuánto debería dar? ¿Y quién me ayudará cuando yo lo necesite? Estas no son preguntas morales abstractas. Son problemas prácticos que los seres humanos han enfrentado a lo largo de la historia evolutiva. Y plantean una pregunta fundamental sobre la evolución y el comportamiento humano: ¿cómo deciden las personas a quién ayudar cuando cooperar es costoso y los recursos son limitados? Dos respuestas clásicas son el parentesco y la reciprocidad. Pero estas explicaciones dejan asuntos importantes sin resolver. Las personas a menudo cooperan con personas nada aparentadas, incluso cuando la reciprocidad es incierta. Además, en poblaciones de fertilidad natural, las personas suelen tener más parientes disponibles (por ejemplo, primos, hermanos, cuñados) de los que podrían permitirse ayudar de manera practica o realista. Entonces, ¿qué mecanismos psicológicos permiten a las personas navegar de forma flexible estos dilemas sociales? A partir de los mercados biológicos y de la teoría de la interdependencia de aptitud (fitness), mis colegas y yo sostenemos que una parte clave de la respuesta reside en cómo las personas perciben el destino compartido.

El destino compartido es el grado en que las personas creen que los resultados de otra persona —buenos o malos— afectarán los propios. Si tu hermano se enferma, eso puede afectar directamente tu carga de trabajo, tu seguridad alimentaria, tu fortaleza de grupo o tus necesidades de cuidado infantil. Si tu compañero de pesca tiene éxito, tú también podrías comer mejor. En contraste, si un conocido pasa por dificultades, su infortunio puede tener pocas consecuencias para ti. Desde una perspectiva evolutiva, el destino compartido es una estimación psicológica de la interdependencia de aptitud: el grado en que la supervivencia y la reproducción de dos individuos están ligadas. La idea central es simple: las personas deberían estar más dispuestas a ayudar a quienes comparten con ellas una apuesta positiva de aptitud. Pero ¿de dónde provienen estas percepciones y qué señales utilizan las personas para inferir el destino compartido?

Para abordar esta pregunta, realicé trabajo de campo entre los Mayangna, una sociedad indígena de pequeña escala en el norte de Nicaragua. Los Mayangna dependen principalmente de la horticultura, complementada por la pesca, animales domésticos, caza limitada y trabajos asalariados escasos o estacionales. Para los Mayangna, como para muchas sociedades de subsistencia, riesgos recurrentes como la escasez de alimentos, la enfermedad y los desastres forman parte de la vida cotidiana. En ausencia de seguridad laboral asalariada o de apoyo institucional, la cooperación suele ser la mejor póliza de seguro disponible frente a los múltiples desafíos de la vida. Entrevistamos a 146 adultos y les preguntamos sobre sus relaciones con tres tipos de personas: un conocido, un primo y un hermano. Para cada relación medimos: (1) diez fuentes de interdependencia, como el parentesco, la co-residencia durante la infancia, comer juntos, cultivar, cazar o pescar, compartir trabajo (es decir, construcción del hogar), religión compartida y experimentar desastres juntos (es decir, inundaciones y huracanes); (2) el destino compartido percibido, medido con ítems como “Lo que es bueno para [esta persona] es bueno para mí” y “Cuando [esta persona] tiene éxito, me siento bien”; y (3) la cooperación en siete dominios relevantes para la aptitud, incluyendo dar carne o pescado, compartir cosechas, cocinar o preparar alimentos, ayudar con el cuidado de los niños, colaborar en tareas de construcción del hogar, asistir después de desastres y ayudar a alguien perjudicado por personas externas.

Luego abordamos una pregunta directa: ¿qué fuentes de interdependencia de aptitud moldean las percepciones de destino compartido, y esas percepciones guían la cooperación? Como cabría esperar, no todas las señales de interdependencia son iguales. La mayoría de las fuentes de interdependencia se correlacionaron con un mayor destino compartido entre participantes (es decir, aquellos que mencionaron altas fuentes de interdependencia en general mencionaron percepciones de destino compartido más alto). Sin embargo, al examinar cuáles señales eran más diagnósticas a través de las relaciones dentro de cada participante, destacaron tres: el parentesco, la comensalía (comer juntos de manera regular) y las actividades de subsistencia compartidas (es decir, sembrar y cosechar, y cazar o pescar). Estas no son señales arbitrarias. Son atributos o actividades que vinculan directamente los resultados de las personas. El parentesco conecta intereses de aptitud a largo plazo; mientras que comer, cultivar y recolectar juntos no solo permite compartir riesgos, sino que también puede comunicar a nuestros socios que valoramos su bienestar por encima del de otros. De manera importante, otras señales plausibles —como la co-residencia en la infancia o compartir la misma religión— no se asociaron con el destino compartido una vez que se tuvieron en cuenta las fuentes centrales. Esto sugiere que el destino compartido no se trata simplemente de monitorear la cercanía social o la afiliación grupal per se. Se trata de monitorear señales de interdependencia relevantes para la aptitud.

El destino compartido percibido, a su vez, se asoció fuertemente con la cooperación. Las personas fueron más propensas a ayudar a quienes percibían con mayor destino compartido en todos los dominios que medimos, desde compartir alimentos hasta ayudar después de inundaciones. Además, el destino compartido medió estadísticamente la asociación entre parentesco y la ayuda. Esto sugiere que el destino compartido es un mecanismo psicológico clave mediante el cual el parentesco estructura gran parte de la cooperación observada en las comunidades Mayangna —y quizás también en otras sociedades humanas. Finalmente, quisimos probar si el destino compartido se asociaba con conductas costosas reales, no solo con autoinformes. Realizamos un estudio de seguimiento con un subconjunto de participantes y les dimos una elección real: quedarse con el dinero o usarlo para comprar arroz para una persona en específico. El destino compartido se asoció fuertemente con la decisión de renunciar al dinero para ayudar a la otra persona.

Aunque una interpretación de este trabajo es que nuestros hallazgos simplemente reafirman la importancia del parentesco para la cooperación, sostenemos que esa conclusión sería demasiado estrecha. La implicación más amplia es que los humanos parecen poseer un sistema psicológico flexible para estimar señales de interdependencia de aptitud, que integra múltiples señales de acuerdo con las oportunidades ecológicamente relevantes. En sociedades donde la cooperación está ampliamente estructurada por el parentesco, como ocurre entre los Mayangna, el destino compartido debería seguir de cerca al parentesco. En sociedades donde la aptitud depende de la cooperación con personas poco o nada aparentadas —como amigos, vecinos, o socios de trabajo o de intercambio— el destino compartido debería monitorear señales correspondientes, incluyendo una historia de trabajo compartido, la puesta en común de riesgos o incluso la cooperación misma.

En suma, el destino compartido percibido puede ofrecer una solución simple a los dilemas de cooperación: ayudar a quienes tienen su bienestar ligado al propio. Seguir esta heurística permite a las personas mitigar costos de oportunidad, protegerse de los costos de la deserción y obtener los beneficios de la cooperación. Comprender cómo las personas llegan a percibir el destino compartido con otros puede ayudarnos a entender no solo la cooperación en sociedades de pequeña escala, sino también la amistad, el apoyo social y la acción colectiva frente a desastres de mayor escala. Cuando las personas sienten que sus futuros están entrelazados, la cooperación sigue. Cuando ese sentido de destino compartido se erosiona, también lo hace la disposición a ayudar. Comprender el destino compartido puede ser tan relevante para entender cómo las personas en sociedades modernas enfrentan la sombra de los desastres (por ejemplo, el cambio climático o los conflictos intergrupales) como para las sociedades de pequeña escala del pasado y del presente.

Link to article

Diego Guevara Beltran, Jessica D. Ayers, Lee Cronk, Daniel P. Balliet, Jeremy Koster, & Athena Aktipis. (2026). Sources of fitness interdependence associated with shared fate and cooperation in a small-scale horticultural society. Evolution & Human Behavior, 47, 106802.