Inclusive Fitness

It’s summertime, which means you might be spending more time outside enjoying nature, breathing in the fresh air, and taking notice of all the creatures around you, big and small. If you’re walking barefoot, hopefully you’re also on the lookout for bees. If you’re unlucky, a tiny bee might spot you as a potential intruder to her hive and launch into action. She could sting you, attempting to ward off the threat by delivering a powerful blow—and then die shortly after. From an evolutionary standpoint, this seems like a terrible trade. She gives up her one shot at survival and reproduction just to defend others. Why would natural selection favor such apparent self-sacrifice?

This seemingly contradictory behavior baffled even Charles Darwin, who worried that behaviors like this could derail his entire theory of evolution. The answer came much later with the concept of inclusive fitness. This theory proposes that evolution doesn’t just favor individuals who maximize their own reproduction; it also favors those who help close relatives, because kin share many of the same genes. So even if a bee doesn’t reproduce herself, by protecting her genetically similar sisters (especially the egg-laying queen), she’s still helping her genes get passed on.

While this idea was first developed to explain puzzling behaviors in animals, from suicidal bees to babysitting meerkats, it turned out to have profound implications for understanding human behavior, too. Why do grandparents invest so much in grandchildren? Why are people more likely to risk their lives for siblings than strangers? Inclusive fitness provides a framework for answering these questions. It reshapes our understanding of family ties, caregiving, and cooperation, showing that our deepest social instincts may be rooted in the logic of shared genetic success.

Hamilton’s Rule (rb > c): A mathematical inequality stating that altruism evolves when the genetic relatedness (r) between the actor and recipient, multiplied by the benefit to the recipient (b), exceeds the cost to the actor (c). Introduced by evolutionary biologist W. D. Hamilton, this rule provides a cost–benefit framework for understanding social behaviors.¹

Coefficient of Relatedness (r): A measure of how genetically similar two individuals are, on average. It represents the probability that a gene in one individual is also present in another due to common ancestry (e.g., siblings have r = 0.5).¹

Cost (c): In Hamilton’s rule, this refers to the reproductive cost to the individual performing the altruistic behavior. It's the downside that must be outweighed by the benefits to others.¹

Benefit (b): The reproductive advantage gained by the recipient of altruistic behavior. For altruism to evolve, this benefit (when weighted by relatedness) must be larger than the cost to the actor.¹

Altruism (Biological): Behavior that reduces an individual's own reproductive success while increasing the success of another. In evolutionary biology, altruism is puzzling because it seems to contradict natural selection unless explained by mechanisms like kin selection.¹

Kin Selection: A form of natural selection that favors traits that increase the reproductive success of relatives, even at a cost to the individual. Inclusive fitness is the mathematical formalization of this idea.¹

Haplodiploidy: A genetic system found in insects like bees, ants, and wasps, where males are haploid (one set of chromosomes) and females are diploid (two sets). This system can lead to high relatedness among sisters and was thought to favor the evolution of eusociality.²

Eusociality: An advanced form of social organization characterized by cooperative brood care, overlapping generations, and a division of labor into reproductive and non-reproductive castes (e.g., worker bees). Its evolution was central to debates around inclusive fitness.²

Weak Selection: A condition where evolutionary changes happen in small, incremental steps. Under weak selection, simplifying assumptions in models like Hamilton’s rule become more accurate and useful.²

Group Selection: A theory proposing that natural selection can act at the level of groups, not just individuals. Groups with more cooperative members may outcompete others, even if cooperation is costly at the individual level.²

Kin Recognition: The ability of organisms to identify and preferentially help their genetic relatives. Hamilton originally considered this as one pathway for altruism to evolve, though it is not always necessary if populations are structured so that individuals interact mostly with kin.¹

Viscous Population: A population in which individuals tend not to move far from where they were born. This limited dispersal increases the chance of interacting with kin, making altruism more likely to evolve even without kin recognition.¹

Nurture Kinship: An anthropological concept that defines kinship based on shared experiences, caregiving, and emotional bonds rather than biological relatedness. It helps explain human social behaviors that don’t align with strict genetic predictions.²

Imagine trying to understand why some individuals in nature act in ways that seem to hurt themselves but benefit others, like the bee stinging an intruder and dying in the process. This puzzling behavior, often called altruism, was a huge head-scratcher for the king of evolution himself, Charles Darwin. Back in 1859, in his groundbreaking book On the Origin of Species, Darwin admitted this was "one special difficulty" that could be "fatal to the whole theory" of natural selection.¹ If evolution is all about individuals reproducing more, why would a sterile worker bee sacrifice its life for the hive? Darwin's early thought was that natural selection could work on the family or kin level, not just on the level of the individual, which might explain this phenomenon.

Decades later, the debate was still ongoing. In 1888, scientist Thomas Henry Huxley (ever the pessimist) saw nature as a brutal fight: he believed altruism should be rare and, if it happened, it would only be among close blood relatives.¹ But a little over a decade later, the naturalist Peter Kropotkin shed light on the concept, arguing that he saw widespread mutual aid among animals of the same species in Siberia, often without any apparent blood ties. This early back-and-forth was more about philosophy than hard science, with no formal models or experiments to really settle the matter.¹

In the early 1900s, scientist J. B. S. Haldane famously quipped he'd save two brothers or eight cousins, but not one brother or seven. This gene-counting logic was a clear precursor to inclusive fitness, suggesting a gene for altruism could spread if it benefited enough relatives, even if the altruist paid a cost. But he never developed a general equation or theorized how selection would optimize such altruistic rules.¹ Researcher Ronald Fisher, who extensively studied genetic relatedness, briefly touched on how traits like distastefulness in insect larvae could evolve if their death saved many siblings, benefiting the family's genes.1 He was on the verge of a mathematical model, but didn't take the next step. 

Even Sewall Wright, who created the "coefficient of relatedness" (r), didn't connect it to a mathematical model of altruism, despite working on group selection.1 Why all the near misses? Perhaps they lacked an economic-based approach to evolution, or they didn't think about costs and benefits as variables in a way that would lead to a mathematical model.

Then came W. D. Hamilton, who truly formalized Darwin's century-old idea. In 1963, in The American Naturalist, he published "The evolution of altruistic behavior."¹ He declared that animal behavior is just as much a product of evolution as physical traits are, but noted altruism (promoting others' advantages at one's own expense) was a challenge. The next year, he expanded on this with Hamilton's rule (rb > c). Hamilton introduced his pivotal model, using Wright's r (genetic relatedness) and factoring in the benefits (b) of altruism to the recipient and the costs (c) to the altruist. The rule states that natural selection favors altruism when the relatedness (r) multiplied by the benefit (b) to the recipient is greater than the cost (c) to the altruist (r × b > c).¹ 

This was a pivotal moment for the field, allowing for an economic, cost-benefit analysis of altruism. To understand how this works in action, think of the theoretical situation in which harm comes to a sibling: according to Hamilton’s formula, altruism is favored if the benefit to a brother or sister (r = 0.5) is greater than twice the cost to the altruist.¹

Hamilton initially proposed two ways organisms could meet this rule he’d created. The first was kin recognition (active discrimination).1 In this model, organisms might evolve to directly recognize relatives and direct altruism towards them. He even speculated about supergenes that could identify copies of themselves in others.1 The second situation was in viscous populations (spatial cues), where individuals don't move far from their birthplace, and social partners are often already closely related. So, even indiscriminate altruism could be favored because it would usually benefit kin.¹,²

In 1970, George Price came across Hamilton's work but was disturbed by the fact that it seemed to scientifically analyze goodness. He later realized Hamilton had underestimated the power of his own equations, and Price developed a new mathematical way to model evolutionary change, which was used to further show that if relatedness within groups is lower than in the overall population, then even spiteful behavior could evolve.¹ Price's model was published alongside a complementary article by Hamilton in Nature in 1970. Hamilton later refined his theory, accepting Price's point that a general regression coefficient was a more accurate measure for his rule, which also helped clarify the link between inclusive fitness and multi-level selection.¹ He also retracted his supergene speculation about innate kin recognition as a primary mechanism, noting that inbreeding avoidance is a different selective pressure (which anyone who has a sibling can attest to).¹

Hamilton proposed that the unusually high genetic relatedness among sisters in haplodiploid insects (like ants, bees, and wasps) could explain the frequent emergence of eusociality: a social structure characterized by cooperative brood care, division of labor, and non-reproductive worker castes. This became known as the haplodiploidy hypothesis.² But by the 1990s, cracks began to form in this idea, as researchers discovered eusocial species like termites and snapping shrimp that use standard diploid sex determination (like humans), suggesting that haplodiploidy wasn’t the universal driver of eusocial evolution it was once thought to be.² 

The debate escalated in 2010 when several researchers published an article in Nature, arguing that the prevailing theory of inclusive fitness was deeply flawed.²  They claimed that standard natural selection, without any need for inclusive fitness theory, offers a clearer, more general explanation for the evolution of eusociality. What’s more, inclusive fitness only works under very narrow conditions and fails to capture the complexity of real-world evolution. Even though Hamilton’s rule says a behavior will evolve if the genetic benefits outweigh the costs, it almost never holds in real populations. In the critics’ view, relatedness isn’t a cause of eusociality, but a consequence. Instead, they concluded that a gene-centered, population-genetics approach was sufficient, and that inclusive fitness could be discarded altogether.² 

Unsurprisingly, this stirred intense debate yet again. Leading evolutionary theorists like Jonathan Birch responded by arguing that inclusive fitness remains a powerful framework for understanding social evolution.⁵ That’s because it holds up pretty well when evolution happens gradually, in very small steps. This slow-and-steady approach, known as weak selection, has a long history in biology and helps make inclusive fitness a useful tool for understanding how traits improve over time.

A common point of confusion is how inclusive fitness applies to people. Some psychologists take it to mean we’re naturally more kind or helpful to our close genetic relatives. But many biologists say that’s not quite right. Because inclusive fitness is meant to explain how traits evolve on average over time, it is more applicable to real life, where we often help those we grow up with or live near. That may be people who are related to us, but not always. So it's not just about consciously spotting our kin, but about how patterns of behavior evolve in environments where kin are usually nearby. 

This fits with a well-known concept in anthropology called nurture kinship, which says that human relationships are often built on shared care and experience, not just on being biologically related. Think about how people often treat those they live with or care for like family, even if there’s no genetic connection (sometimes called our chosen family). Most social anthropologists now agree that emotional bonds matter more than blood ties when it comes to how we define kinship.³

Although scientists continue to debate whether kin selection (which focuses on helping relatives) and group selection (which looks at how groups evolve) are really separate ideas or just different ways of describing the same thing, in many cases, they lead to similar results. Some researchers argue that group-level models are easier to use in situations with lots of moving parts, like when individuals interact in unpredictable ways or live in changing environments.⁵ Supporters of inclusive fitness believe it still applies in most situations, even complex ones, while others argue that group selection can sometimes explain patterns that inclusive fitness can’t.

Charles Darwin

A British naturalist, biologist, and the father of evolutionary theory. Darwin was the first to point out that altruistic behaviors, like sterile worker bees sacrificing themselves for the hive, posed a serious challenge to his theory of natural selection. He speculated that selection might act on the family or group level to explain these puzzling traits.¹

Thomas Henry Huxley

A biologist and anthropologist known for his defense of evolution, Huxley believed nature was a battlefield. He saw altruism as rare and likely to occur only among close blood relatives, a view that foreshadowed kin selection thinking.¹

Peter Kropotkin

A Russian naturalist and anarchist who pushed back against Huxley’s bleak view. In Mutual Aid (1902), he described cooperation and altruism as widespread in nature, even among unrelated individuals. Though not a mathematical biologist, his observations laid the philosophical groundwork for questioning purely competitive models of evolution.¹

J. B. S. Haldane

A geneticist who famously joked he'd “lay down his life for two brothers or eight cousins.” This was one of the earliest intuitive statements of what would become inclusive fitness: the idea that genes can spread by helping relatives, even at personal cost.¹ 

Ronald Fisher

A British mathematician, biologist, and one of the founders of population genetics. Fisher wrote about how traits like self-sacrificial behavior in larvae might evolve if they helped enough relatives. He edged toward inclusive fitness but didn’t build a formal model.¹

Sewall Wright

A mathematical biologist who developed the coefficient of relatedness, which is a measure of genetic similarity between individuals. Although he didn’t apply it to altruism directly, his work provided a key ingredient for Hamilton’s later model.¹

W. D. Hamilton

A British evolutionary biologist and essentially the architect of inclusive fitness theory. Hamilton formally explained how altruism could evolve using a cost-benefit equation (rb > c), now known as Hamilton’s rule. His work revolutionized the study of social behavior and is considered one of the major breakthroughs in evolutionary biology.¹

George Price

A population geneticist who initially reacted with moral discomfort to Hamilton’s ideas. He later refined Hamilton’s equations with what’s now called the Price equation, showing how both altruism and spite could evolve. His work linked kin selection to broader frameworks like group selection.¹

Jonathan Birch

A philosopher of science who strongly defends inclusive fitness theory. He argues that Hamilton’s rule, especially in its generalized form, remains a powerful tool for organizing and interpreting models of social evolution, particularly under conditions of gradual evolutionary change (weak selection).⁵

Inclusive fitness theory revolutionized how scientists understand social behavior—not just in ants and meerkats, but across the entire animal kingdom, including humans. By shifting the focus from individual survival to genetic success shared among relatives, it provided a powerful framework for explaining cooperation, caregiving, altruism, and even conflict. From sterile insect castes to grandparental care, inclusive fitness has become a foundational concept in evolutionary biology, helping researchers decode the deep evolutionary logic behind why organisms help those around them.

Hamilton’s rule ruled evolutionary biology

W. D. Hamilton's rule is a true superstar in the field of evolutionary biology, and it’s helped us to understand the evolution of social behaviors, especially altruism. Remember, the rule is concise: rb > c. It suggests that a gene for an altruistic trait will spread in a population if the genetic relatedness (r) between the altruist and the recipient, multiplied by the benefit (b) the recipient receives, is greater than the cost (c) incurred by the altruist.¹

This was a major breakthrough because for decades, biologists (Darwin included) had wrestled with the paradoxical existence of altruism: Why would any organism risk (or even sacrifice) its own survival for others? Hamilton’s rule finally explained how seemingly selfless acts could evolve and how genes influencing altruism could spread if they benefited relatives who were likely to carry the same genes. In developing his theory, Hamilton helped launch an entirely new way of thinking about evolution, one focused not just on individuals, but on genes as the unit of selection. Hamilton’s original papers from 1963 and 1964 are still widely cited, and his work gave researchers a concrete way to study social behavior and kinship in everything from ant colonies to primate groups. Scientists began tracking kinship ties, predicting patterns of cooperation and conflict, and even using Hamilton’s rule to explain how much different individuals invest in their relatives.¹

Even as the rule became foundational, scientists kept probing its boundaries, because it turns out that this elegant equation works best under specific conditions, like when evolutionary changes happen slowly (what’s called weak selection) or when individuals interact in very simple, predictable ways. Of course, nature is often messier than that, and the "classical" version of the rule, where cooperation evolves if b × r > c, almost never holds up perfectly in real-world populations. That’s because natural populations aren’t static, and life experience may tell you that individuals don’t always interact in neat pairs. Costs and benefits can shift, relatedness can be complex, and other factors (like spatial structure or group-level dynamics) can play a huge role. As a result, modern versions of Hamilton’s rule tend to use more abstract or generalized forms. These still capture the spirit of the idea that genes choose beneficial copies of themselves, but with more mathematical flexibility to account for real-world complexity.

Understanding animal behavior 

This theory has fundamentally reshaped how scientists understand animal behavior, particularly the evolution of cooperation, caregiving, and social living across species. Hamilton’s framework, for example, gave a clear explanation for why sterile worker insects would devote their lives to helping the queen reproduce. Since these workers share a large portion of their genes with the queen’s offspring, their efforts still help pass on their genetic material, even if they never reproduce themselves.² This was a breakthrough in understanding eusociality, one of the most extreme forms of cooperation in nature.

In species like scrub jays, meerkats, and naked mole-rats, non-breeding individuals often help raise the young of close relatives. That’s because by helping their kin raise offspring, helpers increase the chances that shared genes are passed on. These “helpers at the nest” behaviors are more common when helpers are closely related to the breeders. In wolves and wild dogs, packs often consist of extended families, and inclusive fitness helps explain why individuals cooperate in hunting and caring for pups. The benefits of group living—like increased survival of pups and more successful hunts—flow back to each individual indirectly through shared genetic success.

The theory also explains who animals choose to fight (or avoid fighting): many animals are less aggressive toward close kin, which makes evolutionary sense. Harming a relative could reduce your own inclusive fitness, and scientists have observed this in species ranging from monkeys to fish. Even more subtle behaviors like recognition of kin, food sharing, or warning calls (more on that later) have been understood through the lens of inclusive fitness. Just as in humans, many animal species seem to show more empathy or support toward their relatives.

Unpacking complex human behavior

Inclusive fitness theory hasn’t only shed light on the behavior of animals in the wild; it’s also been hugely influential in helping scientists understand the evolution of human social behavior, especially acts of cooperation, caregiving, and even self-sacrifice. Before the theory was established, it was hard to explain why people would go out of their way to help others, especially when it seemed to come at a personal cost. Inclusive fitness reframed the question: instead of asking why individuals act selflessly, it asked how such behaviors might help genes succeed. The key idea is that individuals can pass on their genes not only through their own offspring but also by helping close relatives survive and reproduce because those relatives likely carry some of the same genes.⁴

In humans, this helped explain family bonds. From inheritance patterns to child-rearing practices across cultures, so many human societies emphasize family ties. As we’ve discussed, people are generally more willing to help close kin like siblings or children than distant relatives or strangers.³ From an inclusive fitness perspective, helping your child or sibling can still be a form of genetic “success.” Older adults, especially grandmothers, often invest in their grandchildren long after they’ve stopped reproducing themselves, which may help explain, from a biological perspective, why women’s lifespans extend far past the onset of menopause. If you’re lucky enough, you may have grown up with such a grandparent; maybe they baked cookies for you, or read you bedtime stories, or made sure you always had an extra layer on before going out in the cold. Inclusive fitness frames this behavior as a strategy that boosts the survival of shared genes (you’re more likely to survive if you’re warm and well-fed)! 

Importantly, the theory also provided a baseline to explore why humans help non-relatives, which often requires different explanations, like reciprocity, reputation, or cultural norms. Obviously, family structures have changed a lot since the first humans evolved, and the rise of adoption, same-sex marriage, and chosen families have proven that non-blood-related bonds can be just as strong as those based on shared genes. But by grounding human behavior in evolutionary logic, inclusive fitness helped shift the study of human cooperation from philosophical speculation to a more scientific, testable framework.⁴,⁵ It’s not the whole story, but it gave us one of the first solid pieces of the puzzle.

There’s an ongoing debate in evolutionary biology about how cooperation and altruism really evolve. Some scientists argue that the traditional "worker-centered" view, focused on why individuals give up their own opportunities for reproduction, misses the bigger picture, and that a simpler, "gene-centered" perspective rooted in traditional natural selection may better explain these behaviors without needing the extra complexity of inclusive fitness theory.

The worker-centered view versus the gene-centered view

Not everyone agrees on how altruism and social behaviors evolve, and major critics of inclusive fitness say that the theory focuses too much on the worker. Why would an organism give up its chance to reproduce (a central tenet of evolutionary success) to help someone else? Think of the many eusocial insect societies like ants, bees, and termites, where sterile individuals forgo reproduction to help the queen raise more offspring. Some researchers propose an alternative theory to inclusive fitness that’s more straightforward: go back to standard natural selection. 

From a gene-centered perspective, altruism isn’t a paradox at all, because a worker bee that may seem like she’s being altruistic may simply be doing what her genes (or more precisely, her queen’s genes) have built her to do.²  In this view, workers aren’t individual strategists making personal tradeoffs; they’re part of the queen’s reproductive machinery, shaped entirely by her genetic blueprint. So, rather than labeling the female who stays behind as “altruistic” and the one who leaves as “selfish,” they argue both are just necessary roles in a reproductive system. And whether or not these roles evolve depends not on relatedness per se, but on whether the colony strategy works better than going solo.²

In debates about social evolution, there's a longstanding assumption that genetic relatedness is the primary driver of altruism. After all, we’re more likely to help our siblings than strangers, right? But some researchers argue that this focus on kinship can actually miss the bigger picture, and that relatedness isn’t what causes cooperation, but is instead what results from it.²  Think about it this way: in both insect colonies and human societies, close-knit groups may form because they work well, not necessarily because they started out closely related. For example, in early human history, certain ecological conditions (like abundant, defendable resources) may have favored small, stable groups where cooperation paid off. Helping others in your group by sharing food, defending territory, or caring for children wasn’t necessarily about selfless generosity; it was a smart survival strategy.⁴

Over time, people who stayed in those groups were more likely to be related simply because they stayed. That is, cooperation came first, and kinship followed. So while familial bonds can strengthen cooperative behaviors, they may not be the root cause.⁴ Instead, the structure of the environment and the dynamics of group living could be doing the heavy lifting. This theory follows much of the research on how similar traits like empathy may have evolved in early humans. 

Oversimplification of our dynamic world

Some scientists argue that inclusive fitness theory, while historically important, has limits, especially when we try to apply it broadly to humans and other complex organisms. Inclusive fitness is a specific mathematical tool, not a general explanation for every kind of evolutionary change, and it relies on narrow assumptions. For the theory to hold up, certain conditions must be met, like simple social interactions, weak selection pressures, and highly structured populations.⁵ Clearly, this doesn’t describe real-life human societies, which are messy, dynamic, and full of competing influences.

Inclusive fitness theory also oversimplifies relatedness, which is complex and can take on many different meanings. Measuring genetic relatedness alone doesn't always tell us much. Just think about the sibling sets you know who are theoretically more genetically similar to each other than anyone else on the planet, but could not be more different from one another (maybe you’re thinking of you and your own sibling). On a societal level, you can have two human communities with similar family structures but wildly different norms around sharing, cooperation, conflict, religion, and virtually every other value that makes up our social identity. 

Overcomplicates by obscuring 

On the flip side of this, inclusive fitness can sometimes overcomplicate things. The theory usually ends up telling us the same thing that traditional evolutionary models would, so it’s possible that this extra layer of framework is making things unnecessarily complex. By focusing so heavily on kinship, for example, the theory of inclusive fitness might obscure more than it reveals, because we may overlook the cultural, ecological, sociological, and psychological mechanisms that shape how humans actually behave. This can leave critics with the sense that inclusive fitness is outdated and reminiscent of old astronomical models that tried to explain planetary motion with overly complex math.⁵ Although both were useful once, these models are no longer the best tools we have.

So, is inclusive fitness still useful? Yes, but like all models, it works best when we understand its limitations. As we continue to explore how and why humans cooperate, we’ll likely need to draw on multiple perspectives, each offering a piece of the puzzle.

Squirrels’ survival screams

When I lived in London, I loved my bedroom. I had huge windows with natural light that looked out onto the canopies of the trees surrounding my flat. Everything was green and peaceful; usually, I’d wake up to the chirps of warblers or even the cooing of a pigeon. Other times, I’d wake up to screaming squirrels. Not only was this sound an anxiety-inducing way to come into consciousness, but once I figured out where it was coming from, I worried for my little squirrel friends who lived right outside my window. At first thought, the piercing scream of a ground squirrel might seem like a death wish. Why on earth would a small, tasty mammal draw attention to itself when predators like our neighborhood coyotes are nearby? 

But this behavior, far from being irrational, turns out to be a smart evolutionary move, especially when viewed through the lens of inclusive fitness theory. The first thing to understand is that ground squirrels live in social colonies, and the females tend to stay close to their birthplace (they’re a viscous population).⁶  Meanwhile, males usually disperse, and as a result, adult females are typically surrounded by close kin: sisters, mothers, daughters, and nieces. Males, by contrast, often live among strangers. Fieldwork has revealed that females are significantly more likely to give alarm calls than males. The likelihood of calling increases when close relatives are nearby. Squirrels even tailor their calls based on the type of predator, using distinct vocalizations for aerial threats (like hawks) versus ground ones (like coyotes). Crucially, the callers usually survive, suggesting that while there may be some cost to the behavior, it’s often quite low.⁶

From an inclusive fitness perspective, the logic is straightforward: calling out when a predator appears may slightly increase the risk to the caller, but it significantly increases the survival chances of relatives nearby. Since those relatives share a substantial portion of the caller’s genes, protecting them helps propagate the caller’s genetic legacy. This isn't self-sacrifice for its own sake; it’s strategic, and the math of evolution favors it when the benefits to kin outweigh the cost to the individual, the tradeoff captured in Hamilton’s rule. The sex difference in calling behavior also makes sense under this framework, since males are less likely to be surrounded by kin, the benefit of calling is much lower. Without nearby relatives to protect, sounding the alarm does little to enhance a male’s genetic success and might only make him a bigger target.⁶ So, evolution has shaped a tendency for males to stay silent, while females are more vocal defenders of the family gene pool.

Thus, even though screaming squirrels might seem like an oddity (and a nuisance when they’re outside your window), they offer one of the clearest natural experiments in kin selection and inclusive fitness. What looks like altruism is, at its core, a finely tuned evolutionary strategy: one where genes are looking out for themselves, even across multiple bodies. So next time you hear a squirrel scream, don’t assume it’s panicking. It may be making a calculated, evolutionarily honed choice, risking a little to protect a lot, especially when family is nearby.

Helping cousins: testing the influence of danger, relatedness, and emotional closeness

By now, we’ve established that inclusive fitness theory explains why individuals are more willing to help those to whom they are genetically related, as this indirectly promotes the survival and propagation of shared genes, including a hypothetical "kin altruism gene."³  While previous studies have shown that people help family members more than non-family members, and genetically closer relatives more than distant ones, this 2019 research was the first to look at the precise influence of a recipient's reproductive potential, without using common proxies like age, health, or wealth, which can infer reproductive value.³

The researchers used three smaller studies, in which all participants were presented with hypothetical scenarios where they had to decide the order in which they would help various relatives. The scenarios varied in cost of helping: either a mundane task (e.g., helping with errands) or a life-or-death situation (e.g., escaping a burning building). In the first study, female cousins were described as "preparing to conceive a biological child" (high reproductive potential) or "preparing to adopt a child" (low reproductive potential), and the participants either had a high or low emotional closeness (high vs. low) with them. In life-or-death situations, participants significantly prioritized helping cousins preparing to conceive a biological child over those preparing to adopt, which aligns with inclusive fitness theory, and in mundane situations, there wasn’t any significant difference. Emotional closeness had a powerful and consistent effect: cousins described as emotionally close were helped significantly earlier across both mundane and life-or-death scenarios, regardless of reproductive potential.³

The second and third studies refined the cousins’ reproductive potential, emotional closeness, and relatedness. Interestingly, in mundane scenarios, cousins with low reproductive potential were helped earlier than those with high potential, and participants often explained that this reversal was likely due to sympathy for their cousins less likely to conceive, a common motivator for helping with low-cost tasks. Overall, cousins were helped earlier than friends, and targets likely to conceive were generally helped earlier than those unlikely to conceive, especially in life-or-death situations.³

These studies provide compelling experimental evidence for the causal effect of reproductive potential on helping intentions, particularly when an individual's survival is at stake and the cost of helping is high. It's also crucial to note the consistent and powerful role of emotional closeness across all scenarios, independent of genetic relatedness or reproductive potential. This suggests that while evolutionary drives are at play, our feelings for others remain an undeniably important factor in kin altruism.³

The research also highlights the context-dependency of helping motivations. When the cost of helping is low, factors like sympathy, social norms, or practical considerations seem to exert a greater influence than reproductive potential.³ While these findings are robust, it's important to acknowledge that the studies relied on participants from a Western context, involved only female cousins, and described hypothetical scenarios, which may not perfectly capture real-world decision-making complexities.

What is Altruism? 

Inclusive fitness theory plays an important role in our understanding of altruism. But what exactly is altruism? This article not only gives an in-depth definition of the concept, but also discusses real-life examples of altruistic behavior and some of the current research on our inclination to help others. 

Selfish Altruism: A Win-Win? 

Although altruism may be a driving force when we choose to volunteer, act sustainably, or generally help others, selfishness and an interest in preserving our self-image may also play a role. This article explores research that suggests that those who act altruistically may also be acting in their own self-interest. 

1. Dugatkin L. A. (2007). Inclusive fitness theory from Darwin to Hamilton. Genetics, 176(3), 1375–1380. https://doi.org/10.1093/genetics/176.3.1375 
2. Nowak, M. A., Tarnita, C. E., & Wilson, E. O. (2010). The evolution of eusociality. Nature, 466(7310), 1057–1062. https://doi.org/10.1038/nature09205 
3. Schriver, J., Perunovic, W. Q. E., Brymer, K., & Hachey, T. (2019). Do Relatives With Greater Reproductive Potential Get Help First?: A Test of the Inclusive Fitness Explanation of Kin Altruism. Evolutionary Psychology, 17(3). https://doi.org/10.1177/1474704919867094 (Original work published 2019) 
4. Ågren J. A. (2016). Selfish genetic elements and the gene's-eye view of evolution. Current zoology, 62(6), 659–665. https://doi.org/10.1093/cz/zow102 
5. Birch J. (2017). The inclusive fitness controversy: finding a way forward. Royal Society open science, 4(7), 170335. https://doi.org/10.1098/rsos.170335 
6. Shields, William. (1980). Ground Squirrel Alarm Calls: Nepotism or Parental Care?. American Naturalist - AMER NATURALIST. 116. 10.1086/283651.