In this blog post, we’ll examine how altruistic behavior has been maintained throughout the course of evolution, focusing on the “costly signaling” hypothesis.

Altruistic Behavior and the Costly Signaling Hypothesis

In our daily lives, we often see people helping others—whether by voluntarily donating to support neighbors in need, giving blood and dedicating their time, or willingly volunteering during natural disasters. While these actions entail physical, financial, and psychological costs for the individuals involved, they benefit others. At first glance, the so-called “free-riding”—where one hopes others will act on their behalf without incurring any personal cost—seems like a far more rational behavior. Therefore, from an evolutionary biology perspective, one might think that the number of free-riders in the world would gradually increase, while those who help others would be weeded out and disappear. However, people who engage in altruistic behavior despite having no direct personal benefit have consistently existed. Why have altruistic people, who accept personal losses for the sake of others, been able to survive until now? To find the answer, scholars have proposed various hypotheses. One of these is the “costly signaling” hypothesis, which explains that altruistic individuals were able to evolve because they used altruistic behavior to signal their abilities and qualities. In this article, we will examine this hypothesis through various examples and explore how altruistic humans have been able to survive even in competition with selfish individuals.

Costly Signaling in Animals and Humans

First, let’s imagine we’re observing animals on a safari. As we drive through the safari in a vehicle, we see zebras peacefully drinking water in the distance and giraffes stretching their necks to eat leaves. A little further on, a herd of gazelles is moving through the terrain. Suddenly, a lion bursts out of the grass and charges toward the herd of gazelles. The startled gazelles all appear to be fleeing far away from the lion. But something is strange: one gazelle is leaping high into the air right next to the lion. Witnesses might think, “That gazelle is about to be eaten by the lion.” Interestingly, however, the lion does not chase the gazelle leaping in front of it but instead pursues the other gazelles that are running away quickly. Why doesn’t this gazelle run away, but instead jumps right in front of the lion? And why does the lion give up on the gazelle right in front of him and chase after another one?
Let’s set these questions aside for a moment and look at another scene. On a nearby hill, male peacocks with their magnificent tail feathers spread wide are courting female peacocks. The female peacock surveys the males and selects the one with the largest and most magnificent tail feathers. At first glance, it might seem as though he is chosen simply because of his beautiful feathers. However, large, showy tail feathers are easily spotted by predators and hinder escape, making them a trait that is actually disadvantageous for survival. Nevertheless, the female peacock prefers males with precisely these characteristics. So, could it be that those magnificent tail feathers contain some kind of “signal” we haven’t yet recognized?
Let’s consider the gazelle running in front of the lion once again. From the gazelle’s perspective, to prevent the lion from chasing it, it must demonstrate that even if the lion follows, it will never be able to catch it. But since gazelles cannot speak, they must prove their exceptional running ability through their actions. To do this, it must display a difficult behavior that other gazelles cannot easily imitate. Thus, the gazelle sends the lion a “costly signal”—by leaping high into the air. Why, then, don’t other gazelles perform the same behavior? It’s because individuals lacking sufficient ability cannot send such a signal. Consequently, the lion concludes that only those capable of this behavior possess exceptional abilities and pursues those gazelles instead.
The same applies to peacocks. A male with a magnificent tail sends a costly signal to females, effectively saying, “I have survived despite these conditions that are disadvantageous for survival.” Females use this signal to assess the male’s health and ability and select the male who sends the most costly signal. These two examples demonstrate that only individuals with exceptional abilities can send costly signals, and the greater the cost of the signal, the more reliable the information becomes.
This signaling behavior can also be observed in humans. For example, actions such as showing off one’s strength or purchasing expensive cars and luxury goods to display one’s economic power can also be interpreted as signals intended to demonstrate one’s capabilities. It is a way of proving one’s strength or economic power through behavior. The “costly signaling” hypothesis explains that sending signals unique to oneself to others is one of the reasons altruistic humans can exist. Under this hypothesis, behaviors such as showing consideration for others even in difficult situations, taking the lead in upholding social norms, and sanctioning those who violate norms are also viewed as costly signals sent by humans.
So, from this perspective, why do humans send such costly signals through altruistic behavior? First, it is because altruistic behavior allows them to demonstrate their capabilities. The Mariam people of Australia prepare turtle dishes for every festival; this dish requires a great deal of time, money, and energy, and carries a high risk of injury, so it is by no means an efficient meal. Nevertheless, they willingly prepare this dish because it allows them to demonstrate their exceptional skills, courage, and expertise to the community. Second, altruistic behavior can also work to one’s advantage in the process of mate selection. By performing good deeds for others to showcase one’s character or by donating large sums of money to demonstrate one’s economic power, individuals can display an appeal that others cannot easily imitate. For these reasons, this hypothesis explains that because altruistic behavior ultimately yields certain benefits, altruistic individuals have not been eliminated by natural selection and have survived to the present day.

Significance and Limitations of the Costly Signaling Hypothesis

So far, we have examined the “costly signaling” hypothesis. This hypothesis explains that one reason people help others even in difficult situations and act altruistically even when it results in personal loss is to send signals that reveal their special abilities and qualities. It posits that because humans were able to send costly signals through the specific act of altruistic behavior—and consequently gain benefits in terms of survival, reproduction, and social reputation—altruistic individuals were able to survive even in competition with selfish individuals.
Of course, this hypothesis has its limitations. For example, it fails to adequately explain cases where individuals do not reveal their capabilities to others, such as when donating large sums anonymously or providing support in secret. Nevertheless, it is still regarded as a meaningful hypothesis because it offers a compelling perspective on why people act altruistically even in situations where no repeated interactions are formed.