Ever wonder why we like...do things for each other?

Like why we help our friends move even though we hate doing it?

And, for that matter, why a bee will sacrifice its life by stinging an intruder to protect the hive?

Why would a vampire bat regurgitate blood into the mouth of another bat that hasn't eaten that day?

Why?! That's a very nice vampire bat...I guess.

Eugh!

That's gross in like so many ways.

It turns out that this is actually a pretty big question.

Things like this have been stumping scientists for years.

Charles Darwin thought that altruistic behavior

was a potentially deal-breaking flaw for his theory of natural selection.

If the game was survival of the fittest,

natural selection couldn't possibly favor a behavior that made us less likely to survive.

Or could it?

Darwin studied beehives and realized that since sterile worker bees were helping their blood relatives,

especially the queen,

natural selection might favor altruism within related groups.

A hundred years later, in 1964, a British scientist named William Hamilton

actually came up with an equation to explain this.

He figured altruism could evolve as a trait if:

genetic relatedness, times the benefit of the action, was greater than the cost of the individual.

In other words:

since some behavior is hereditary

the genes responsible for altruistic behavior

could evolve if it's benefit exceeded whatever cost it had for the individual

because it helped the individual's relatives enough to make it worthwhile.

Hamilton called this idea "Inclusive Fitness"

expanding Darwin's definition of fitness

basically how many babies someone is making

to include offspring of other relatives.

Hamilton's ideas were a humongous hit with other scientists

because for starters it explains stuff like ant colonies.

Ants have virtually no personal lives,

everything they do, they do for the good of the colony.

And worker ants share three quarters of their genes.

They're actually more closely related to their sister ants

than they would be to their offspring.

So, according to Hamilton's equation, each individual ant has an exceptionally large genetic stake

in the survival of the colony

But Hamilton's ideas do not explain why some animals help others that they're not even related to.

Take humans, for example. We are deeply social animals, also exceptionally altruistic.

In fact, a lot of scientists think that humans evolved our huge, super smart brains in response to the

overwhelming benefits of engaging in selfless behavior.

Because it turns out sharing and cooperating are very mentally taxing.

Seriously, ask any three-year-old.

But if you think about it, you can see how we evolved to be altruistic for really self-serving reasons.

Like helping a friend move totally sucks, but you do it because you can conceive of a time in the future when that friend will help you move.

Or when that friend will drive out into the country to help you change a flat tire, or whatever.

Same with the bats, barfing some blood into your neighbour insures that someday, if you don't get something to eat,

somebody will come and vomit blood into your mouth,

and they'll expect the same from you in return, and so on, and so on.

Hamilton's equation does not explain that behavior, which complicates things, because equations are nice and comforting to have,

if you're a scientist. But if you can cooperate with others and resist the urge to horde resources,

knowing you'll be rewarded later, you'll be acting a lot like ants, bees and vampire bats.

Which makes me just feel all warm and snuggly all over.

Thanks for watching this episode of SciShow. I hope you have friends good enough to vomit blood in your mouth when you need it.

If you have any questions, comments, or ideas please connect with us on Facebook or Twitter,

or in the YouTube comments below, and if you want to keep getting smart with us here at youtube.com/SciShow,

head on over and subscribe, and we'll see you next time. Goodbye.

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