The Biology of Sacrifice (and Terrorism)

Elin Sild and Tuul Sepp are immune ecology researchers at the University of Tartu’s Institute of Ecology and Earth Sciences. Elin Sild spent last two years as a post-doctoral researcher at Lund University. Tuul Sepp won an award for her popular science articles in the Estonian cultural weekly “Sirp” in 2014.

'Je Suis Charlie' rally in Paris

Terrorism acts often involve self-sacrifice, paradoxically linked to the fear of death. Image credit: Doug / Flickr Creative Commons.

Self-sacrifice for the benefit of others is a powerful symbol. This act is thought of as noble, endlessly auspicious – a supreme case of self-denial. A good indication of the immense power of this symbol is the fact that even now, 2,000 years later, Christmas, the day when Christ was born, is celebrated massively in so many places. Christ’s most impactful act was definitely the salvation of humankind: sacrificing himself so that others could be saved.

Could self-sacrifice also be biologically rational? Could it paradoxically bring profit to the one sacrificing? How is it even possible that the evolutionary process has developed a disposition of such kind?

Recognizing one’s mortality

If we want to study the psychological and biological mechanisms that lead to self-sacrifice, then the term itself must be explained first. The self-sacrifice of a person has been defined as the willingness to accept losses so that personal principles and values could be maintained. 1 As self-sacrifice has been spotted in other species, too, not exclusively in humans, the definition must probably be expanded, so all kinds of actions that seriously harm the actor while (presumably, at least) others capitalize on it can be included.

Jesus dies on the cross to redeem humankind. A schoolboy goes to war to protect his homeland. Kamikazes, the Japanese daredevils, crash their planes into American military objects in World War II. In Lebanon, terrorists blow themselves up in crowded places, as well as in Sri Lanka, Israel, Iraq, and, increasingly, in Western countries as well. Although all these deeds have different levels of violence, as well as (depending on the cultural background) different ethical standards, the inner motivating mechanisms are probably quite similar. In the name of an immortal idea or value, an individual is ready for the ultimate sacrifice – to give up his or her life.

Paradoxically, in the field of psychology, the scientific literature links self-sacrifice to the fear of death. 2 Upon recognizing our mortality, we desperately try to find a way to achieve immortality. Thus, the spectacular act of dying is an aspiration to achieve symbolical immortality while neglecting the physical body.

Yes, my body can die, but my ideas and values will live on, and through them I will live on. The body will eventually die anyway, be it sooner or later. Although we have taken great steps in increasing the average human lifespan, the maximum age (100–120 years) has remained the same, and it’s possible that future scientific discoveries cannot change this inevitability either.

In the study previously referenced (2), people were more willing to sacrifice their bodies in the name of their home country when they were reminded of the mortality of the human body beforehand. A person with a rough background who has seen death and suffering firsthand is probably easier to recruit as a suicide terrorist as well. An interesting question arises here: If people were immortal – although an extremely long lifespan would be held back by diseases and accidents that sooner or later hit a person – would there be any suicide terrorists left? As long as this possibility does not exist, the only way to insure immortality might be a death that is as spectacular as possible.

Ideological and biological immortality

Similarly, the wish to pass one’s genes on to the next generations (“immortal genes”, sailing into eternity through generations of mortal bodies, as described by Richard Dawkins 3) can be seen as an aspiration towards immortality. Let’s call it biological immortality.

On one hand, quests for biological and ideological immortality could be considered opposing claims. The first one implies investing in oneself, the family, sustainable management. The second immortality involves putting everything at stake, giving up every hope for offspring, and not caring about the wellbeing of those close to you. On the basis of this opposition, systems for recognizing terrorists have been developed using statistics about bank-related behaviour to bring suspicious persons into the view of the government. 4 A future terrorist is much less likely to have savings or life insurance.

Still, are the wishes for biological and ideological immortality really that incompatible? Readiness to sacrifice oneself, altruism, and heroic acts are most definitely factors that boost social status and increase admiration and respect from peers, including the opposite sex. According to Amotz Zahavi, an Israeli evolutionary biologist, altruism is an honest signal about the specimen’s quality providing a certain position in the pecking order, and thus increasing one’s chances to breed. 5 Through such a mechanism, natural selection might favour the development and persistence of heroic behaviour.

When a hero-like warlord who always puts himself in danger is much more successful with the opposite sex than his cowardly confrere (one envisions Colonel Buendia, with his 17 sons by 17 admirers from the novel One Hundred Years of Solitude by Gabriel García Márquez), it is clear that self-sacrifice, reaching immortality through a distinguished death, can simultaneously increase the possibility of biological immortality. Heroes don’t always die, and, in case of surviving, the risk taken pays generously.

The spider’s strategy

The need to make oneself stick out for potential mating partners might be the biological explanation why heroic acts are mostly the field of young men. Arguably, the best age for performing heroic acts – when the urge to sacrifice oneself is at its highest – matches the age when scientists, artists, writers, and musicians publish their best works (most often between 25 and 35 also). 6 At this age, the need to stick out is at its highest. The typical suicide terrorist is also a young man. Combined with the promise of self-sacrifice being compensated with numerous young virgins in the beyond, it is easy to see how the suicide bomber’s motivation is mentally related to biological relevance.

The motivation emanating from the greater chances for successful breeding probably is not conscious. Thus, the link between greater fertility and being heroic might not be too translucent. But we have a more explicit example about the relationship between self-sacrifice and procreation. Let’s take redback spiders (Latrodectus Hasselti). 7 To prolong copulation and keep other males from interfering with the mating act, they deliberately twirl their body into the teeth of the female during the process. Approximately 65% of copulations involve such cannibalistic acts.

Redback spider

Redback spiders are known to twirl their body into the teeth of the female to prolong the copulation process and keep other males from interfering with it. Image credit: David McClenaghan, CSIRO / Wikimedia Commons.

This kind of behaviour is useful when the likelihood of a second breeding is very small. More than 80% of male redback spiders die without ever finding a partner. The great chances of getting killed while searching for a mate and the fatherhood-enforcing impact of cannibalism are enough to change self-sacrifice into a useful strategy (for the spiders). In the end there’s not much difference if success at breeding is caused by readiness to be devoured by the female or getting heroically shot on the battlefield. Both ways of acting are examples of the wish to participate in the most important fight of biological life.

The biological profitability of risk behaviour

When it comes to evolution, this kind of biological profitability of risk behaviour might be the deciding factor as to why the behavioural traits leading to self-sacrifice are still present in the population. If self-sacrifice were always successful, the genes causing such behaviour should have become extinct very quickly.

When putting oneself on the limb brings profit to those around you, close people who are likely to carry same genes, it can be evolutionarily useful even when it blocks distribution of the sacrificer’s genes. 8 In cases like this, it is important that members of the sacrificer’s group would gain profit, rather than those outside the group.

An ant species from Malaysia and Brunei (Camponotus saundersi) offers an interesting example about the usefulness of self-sacrifice to the well-being of relatives. The ant colonies, made up of relatives, are protected by “suicide bombers”. Workers of the colony have enlarged salivary glands, full of poisonous chemicals and a special glue-like substance. When the workers sense danger, they contract their abdominals with such force that their salivary glands explode. The sticky, toxic stuff gets out and covers the enemies nearby. It also kills the worker, but it might save the entire colony that would carry on the ant-hero’s genes.

The impact of the “love hormone”

An article recently published in Science magazine described the neurobiological mechanism controlling such self-sacrifice. It is interesting that the main regulator of inner-group altruism (and its evil twin, xenophobia) is thought to be oxytocin – the “love hormone”, mostly known for regulating childbirth and feeding infants. During communication with close ones, oxytocin enters the bloodstream. A greater number of oxytocin receptors is related to better empathy, kindness, and taking others into account. An experiment showed that oxytocin increased aggressiveness with the intent of protecting one’s mates in conflicts between groups.

Great love towards one’s groupmates (be it compatriots, members of the same denomination, or those sharing some other ideals) and the wish to protect them leads to degrading the value of members of competing groups, or even to aggression. Indeed, the effect of the “love hormone” can result in hate, as shown by experiments where oxytocin amplified territoriality and aggressiveness of many species of mammals, as well as stimulate envy following loss in a competition amongst humans, and bragging in the case of victory. 9

Comparing self-sacrifice – a heroic behaviour – to suicide terrorism and looking for biological self-profit in the background of this behaviour might seem cynical, or even immoral. But studying the evolutionary background of why some behavioural traits turned out the way they did does not diminish the heroism of the acts. Heroes’ actions might not be consciously motivated by improving the social status or increasing the breeding potential at all. These are just the mechanisms that maintain a certain behaviour in nature. A manner of acting that does not increase the biological relevance of a specimen would not survive natural selection.

Understanding the psychological and biological mechanisms of self-sacrificial behaviour is necessary when we want to understand the actions of suicide terrorists and find ways to prevent their activities. To members of their own group, they are heroes and protectors, self-sacrificers whose actions are motivated, paradoxically, by love.

The Estonian version of this article initially appeared in “Sirp”.


  1. De Cremer, D., van Knippenberg, D. 2004. Leader self-sacrifice and leadership effectiveness: The moderating role of leader self-confidence. Organizational Behavior and Human Decision Processes 95:140–155.
  2. Routledge, C., Arndt, J. 2008. Self-sacrifice as self-defence: Mortality salience increases efforts to affirm a symbolic immortal self at the expense of the physical self. European Journal of Social Psychology 38:531–541.
  3. Dawkins, R. 1976. The Selfish Gene. New York City: Oxford University Press.
  4. Levitt, S. D., Dubner, S. J. 2011. Superfreakonomics. Harper Perennial, New York City.
  5. Zahavi, A., Zahavi, A. 1997. The handicap principle. Oxford University press, New York, Oxford.
  6. Sepp, T. 2014. Lehtlalindude looming ja loovuse evolutsioon. Sirp, 11.04.
  7. Andrade, M. C. B. 2002. Risky mate search and male self-sacrifice in redback spiders. Behavioral Ecology 531–538.
  8. Inklusiivse kohasuse teooriast ja altruistlikkust koostööst saab lugeda ka Sirbis 14. Augustil 2014 ilmunud artiklist „Koostöö evolutsiooniline paradoksaalsus“.
  9. De Dreu, C. K. W., Greer, L. L., Handgraaf, M. J. J., Shalvi, S., van Kleef, G. A., Baas, M., Ten Velden, F. S., van Dijk, E., Feith, S. W. W. 2010. The Neuropeptide Oxytocin Regulates Parochial Altruism in Intergroup Conflict Among Humans. Science 328:1408–1411.
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