Evolutionary Puzzle of Suicide: when would you kill yourself? In our world, there is nothing worse than killing. And the most terrible thing is a man killing himself.

Some might say enthusiastically: “Why didn’t natural selection eliminate a lot of physical and psychological diseases that drive people to suicide? How do I believe that the aim of evolution is to survive and reproduce if a million people around the world commit suicide, or if someone commits suicide every forty seconds?” [1].

But we do not have to formulate our question in this way before we know if there are evolutionary (adaptive) benefits of suicide.

To begin with, let us discuss the possible reasons of suicide [2]. In the past, the risk of suicide Increased with age, with a greater risk in older men. But there is an exception, in the 1970s the suicide rate among young people increased, especially young males, in high-income countries.

There are distal causes of suicide, which include:

1- Genetic loading (Genetic load is a low mean fitness in a population in relation to another population formed of individuals with optimal genotypes [3]).

2- Personality type (impulsivity, aggression).

3- Defect in the fetal development and perinatal conditions.

4- Early traumatic events.

5- Neurobiological disorders.

And there are proximal causes of suicide, which include:

1- Psychiatric disorders and psychological crises.

2- Physical disorders.

3- Availability of the means of suicide.

4- Watching suicide models (i.e. via social media).

Now, we are excited to know the evolutionary benefits of suicide. But we are in trouble! We, probably,  will not just have to argue that suicide is an adaptation since it ends the whole life. So we might suppose that the person who kills himself may give another person or a group of people some genetic benefits. This is the only way we can understand suicide as an adaptation. Thanks to the evolutionary biologist William Donald Bill Hamilton who raised the twentieth century’s interpretation of the altruistic behavior.

Hamilton thought in his theory of inclusive fitness that altruistic behaviors among people who share a certain percentage of the genes allow these genes to be passed down through the next generations [4]. Kin selection is a special case of inclusive fitness, and it means that natural selection would favor genes responsible for altruistic behavior when the benefits of genetic relatives are greater than the costs of the altruistic person [2]. Mathematically, altruistic behavior will happen if:

r × b > c

Where “r” = the degree of genetic relatedness;

“b” = the reproductive benefit to the genetic relatives;

“c” = the reproductive cost to the altruistic person [5].

But what are the benefits that our biological relatives will receive if we kill ourselves, rather than their tragedy and grief and the spread of their bad reputation in the community?

In this article I will summarize a recent study (2013) published in International Journal of Environmental Research and Public Health entitled “The Evolutionary Puzzle of Suicide” [2].

 1- The Altruistic Suicide Hypothesis:

A professor of psychology and neuroscience and behavior named DeCatanzaro used Hamilton’s theory of inclusive fitness to explain the enigma of suicide. DeCatanzaro thought that the person may commit suicide when the probability of his reproductive success is discouraging, and when he realizes at the same time that his continued existence will reduce the inclusive fitness by interfering with the reproductive success of his genetic relatives [6].

It has been reported that altruistic self-destruction behavior is found in many organisms, from unicellular organisms and parasites to social insects, and it is controversial in human beings. Self-destruction had evolved in a number of social insects as a defensive response against enemies because it has beneficial consequences for the fitness of reproductive individuals of the colony. This means that the insect will expose itself to danger as it is aware that the reproductively successful insects in the colony will survive to reproduce, and then to spread their genes. Suicidal defense behaviors in social insects will occur in three cases: (1) the case of momentary defense—when there is an active enemy, with a greater risk on the defender; (2) the case of preventive suicidal defensive behavior—it means that death is a result of  repair or hiding the nest just for prevention before the enemy’s attack; (3) the case of altruistic self-removal—sick and infected insects leave the nest to prevent the healthy insects from being infected.

What about humans?

Studies have found that the association of burdensomeness (i.e. because of disease or other reasons) towards relatives and low reproductive success (i.e. low reproductive capacity) can lead to negative values. Suicidal thoughts have been correlated with perceived social isolation and burdensomeness.  Furthermore, some people justify their suicidal behaviors as a sacrifice to help their relatives. In the past, Inuit considered suicide an altruistic behavior. Older or ill adults would kill themselves or be killed on their request in times of famine.

In short, this hypothesis sees that suicide is an adaptive behavior in special cases, such as: aging, psychological illnesses, physical illnesses, especially chronic ones and homosexuality.

But if the burdensomeness towards relatives is a reasonable explanation of suicide in elderly, it is difficult to accept that the suicide of young people with good health will improve the fitness of relatives. Instead, it is likely to be an “evolutionary byproduct.”

2- The Bargaining Hypothesis:

Let’s suppose that you are living in a house with ten people and suffering from your parents’ ignorance and their struggle with you. In the worst situation, you may act a suicidal play with guaranteed results, but not deadly. This hypothesis believes that the suicidal behaviors could be “costly cries” for help or an honest sign of need practiced by people who fall in troubles with social groups (family, community, etc). In this case, the attempt for suicide can be a gamble. You may survive and gain what you wanted, or lose your life.

From an evolutionary perspective, this strategy would have developed when the benefits of genes responsible for “signaling strategy” on average are greater than their costs. It is important in this hypothesis that most suicidal attempts are not deadly.

3- The Parasite Manipulation Hypothesis:

It is known that parasites may manipulate the host’s behavior. This manipulation is an adaption for the parasites; it facilitates their transition from one host to another.

One of the most famous examples of this type of adaptive manipulation is what parasite toxoplasma gondii does. This intracellular protozoan has an indirect life cycle. It can live in  mammals as an intermediate host, but it remains immature, and in order to complete its life cycle (attain maturity), it has to be ingested by a predatory definitive host, which are the felids (cats); they are the only mammals able to release their eggs (oocysts) through their faeces.

When another mammal (mouse, for example) becomes infected with T. gondii, it manipulates its behavior, i.e., makes mice less fearful of cats and most attracted to them, so that it can move to the final host to complete its life cycle.

Humans also can be infected by T. gondii and some studies have linked latent infection to suicidal behaviors in humans. Large felids (tigers, leopards, lions) were the primary predators of humans in the twentieth century, and it is likely that these species (in addition to those that have lived in our ancestral environments) have been dangerous predators of early humans.

But the parasite manipulation hypothesis is still surrounded by lots of controversies.

It is interesting in this hypothesis that suicidal behavior may not be adaptive for the subject committing suicide, but for the parasite itself.

“Our economic and cultural landscape has evolved exponentially. Individuals cooperate on an increasing scale and rely less on kin than in the past. We believe that the social welfare developed in many countries dramatically altered the adaptive value of suicidal behavior, which can be understood today as a mismatch: following our environmental change, the adaptive value of suicide has mainly been lost” researchers concluded.

References

[1] http://www.suicide.org/international-suicide-statistics.html

[2] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3881146/

[3] http://www.els.net/WileyCDA/ElsArticle/refId-a0001787.html

[4] http://global.britannica.com/science/inclusive-fitness

[5] http://www.genetics.org/content/176/3/1375

[6] http://blogs.scientificamerican.com/bering-in-mind/is-killing-yourself-adaptive-that-depends-an-evolutionary-theory-about-suicide

Translated from Arabic by: Nour Al-Huda