Fifty years ago this month, evolutionary biologist William Donald Hamilton published a solution to one of biology’s most enduring mysteries: why does altruism exist?

Altruistic behaviours are those where an individual helps others at a personal cost. Altruism is all around us, yet biologists since Charles Darwin had stewed over how such behaviours could ever evolve in the dog-eat-dog world of natural selection.

Hamilton came up with an answer to this quandary, elegantly summarised in the mathematical formula now known as Hamilton’s rule.

So now seems a good time to ask: what did we actually learn from Hamilton, and are his ideas on altruism still relevant half a century on?

The trouble with social insects

The most extreme examples of altruism in nature come from social insects such as bees and ants, in which workers toil endlessly for their colony but don’t reproduce themselves.

Their existence had long puzzled biologists. Darwin considered social insects a potentially “insuperable difficulty” for his theory of natural selection. After all, how can selection work in individuals that are sterile?

Until Hamilton, explanations for the evolution of altruism and cooperation reverted to arguments that it benefited the species. It’s a line still used in third-rate nature documentaries today, and it makes evolutionary biologists cringe. Every single time.

Cooperation may well benefit the species, or even a group, but English biologist Ronald Fisher showed in the 1930s that natural selection doesn’t work like that. When what’s good for the group contradicts what’s best for the individual, the interests of the individual almost always win out.

Nevertheless in the 1960s ideas about group selection were again surfacing, retreading all the old mistakes.

Then in July 1964 Hamilton lopped the wind from their sails with two groundbreaking Journal of Theoretical Biology articles. He described how genes imposing a fitness cost on some individuals (for example sterility in the workers of social insects) can spread, provided they enhance the fitness of relatives that share the same gene.

Hamilton coined the term “inclusive fitness”, which he defined in his own notoriously opaque way:

The social behaviour of a species evolves in such a way that in each distinct behaviour-evoking situation the individual will seem to value his neighbour’s fitness against his own according to the coefficients of relationship appropriate to that situation.

In other words, a worker ant may not have any sons or daughters, but can still produce thousands of brothers and sisters. And in Darwinian terms, that’s just as good as – or even better than – personal reproduction.

Kin selection

Hamilton’s ideas and their subsequent embellishments are now often referred to as “kin selection”, a term coined not by Hamilton but by British evolutionary biologist John Maynard Smith in 1964.

Maynard Smith credited British biologist JBS Haldane as the first to have come up with the notion of kin selection. According to rumour, Haldane declared, in a pub, “I would lay down my life for two brothers or eight cousins”, referring to the fact that our siblings on average share 50% of our genes and cousins 12.5%.

Hamilton contested the Haldane quip. In fact, one of the motivations for Hamilton’s work on inclusive fitness was that Haldane’s work had failed to derive altruism from group selection.

The relationship between Hamilton and Maynard Smith – two of the most influential evolutionary biologists of the 20th century – sadly remained strained until Hamilton’s untimely death in March 2000.

So what exactly is Hamilton’s rule?

Hamilton’s rule specifies the conditions under which a gene causing altruism might enjoy an inclusive fitness advantage. This occurs when the benefits, b, to a related individual exceed the costs, c, to the altruist, discounted by the relatedness, r, between the two:

b > c/r

Evolutionary biologists consider this equation every bit as important as Einstein’s mass-energy equivalence equation (E=mc2).

Hamilton used the evolution of alarm calls in birds as an example. A bird that calls in response to danger risks attracting attention and thus losing its life, as opposed to a bird that remains quiet.

But, according to Hamilton’s rule, such self-sacrificing behaviour can evolve if the benefits to relatives who hear the squawk and gain advance warning (summed by the number of relatives and how closely related they are to the squawker) outweigh the risks to the squawker.

In the past 50 years Hamilton’s rule has been used to explain a plethora of otherwise strange animal and human behaviours:

why do we care for our children?

why do lions form coalitions with their brothers?

why do young birds hang around the nest rearing brothers and sisters rather than breeding themselves?

But does kin selection still hold up?

In 2010, a Nature paper by Martin Nowak, Corina Tarnita and Edward O. Wilson – the latter a once-staunch supporter of kin selection and a mentor to Hamilton – rejected kin selection outright.

Nowak and his co-authors claimed kin selection was simply an unnecessary reformulation of natural selection. They argued that altruistic genes can increase in frequency even when givers and receivers of altruism are unrelated, simply because individuals carrying altruism genes preferentially interact with each other.

They failed to note, however, that their “alternative” mechanism for the evolution of altruism had already been identified by Hamilton and immortalised by Richard Dawkins as the “greenbeard effect”.

Imagine a gene that causes its carrier to grow a green beard. Such a gene will spread if those that carry it preferentially show altruism to other individuals with a green beard.

Nowak, Tarnita and Wilson’s attack on Hamilton has been almost universally dismissed by evolutionary biologists. In 2011 more than 100 eminent scientists signed a letter pointing out the conceptual errors in the paper and documenting the extraordinary predictive power of the kin selection paradigm.

A nicer world

Kin selection provides one of the two strong forces that bind all cooperative enterprise. The other is reciprocity. Think of the first as “blood is thicker than water” and the second as “you scratch my back and I’ll scratch yours”.

We rely so much on reciprocity in contemporary society, with reciprocation made liquid by money and other currencies. But kin selection still gives social life much of its shape.

Genetic relatedness infiltrates human affairs to their very root. From childcare to nepotistic corruption, when all else is equal we favour our relatives over those more distantly related.

It also explains food-sharing practices in traditional societies, the way inheritances are passed down in wills and which grandparent children feel closest to.

The most compelling illustration of kin selection in humans is the work of Canadian evolutionary psychologists Martin Daly and Margo Wilson, summarised in their book The Truth About Cinderella: A Darwinian View of Parental Love.

Just as fairytales warned children about the dangers of the stepmother, so modern data on childhood neglect and violence suggest that a parent’s new partner presents a high risk to the child. These are difficult facts to acknowledge because the vast majority of stepparents provide wonderfully supportive homes to children.

But the key here is what relatedness does to parents. Genetic relatedness more often persuades parents to persist with the hard work of child-rearing, prevents them from abandoning their kids and stays their hand from violent overreactions.

The roots for all these ideas were pioneered 50 years ago by Hamilton, whose work continues to illuminate the origins of our own behaviours.