Hamilton's Rule: Definition & Example

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  • 0:01 Family First
  • 1:02 Kin Selection
  • 1:57 Hamilton's Rule
  • 2:37 Putting Hamilton's…
  • 3:36 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Do you have a nephew that you'd do anything for, even if it disadvantages you? Or maybe you have that close bond with a brother, sister, or even a cousin? It turns out that there is a biological rule for exactly this behavior.

Family First

Let's say you had a child. Chances are you'd do just about anything for that child, wouldn't you? After all, that's your baby we're talking about! Now let's say that you had a niece or a nephew. Again, you'd do a great deal for them, but let's face it, that's not your kid. In healthy relationships, we do these things because we love the people in question. I can tell you from experience that I am willing to do a lot more for my niece and nephew than I would be willing to do for the average toddlers. But is this just because I love them, or because there's some biological factor at play?

As much as I may want to pin it on the affections of their uncle, it turns out that there's actually a scientific explanation. If that wasn't crushing enough, the propensity for an individual to go out of his or her way for the benefit of a related individual can even be mathematically expressed through Hamilton's rule. Named for W.D. Hamilton, it places some cold numbers behind the affection I feel for my niece and nephew.

Kin Selection

First, we have to understand a bigger concept within the entire animal kingdom known as kin selection. In short, kin selection means that we are more likely to do things that further the progress of a shared gene pool, even at the expense of our own well-being. Perhaps the best example of this is the simple honeybee. As you may know, a honeybee sacrifices its life by stinging a large animal.

Based on that, you would think that a bee would have some hesitation about stinging large mammals, whether it's a bear trying to gather honey or a cow that simply wandered too close. And yet, as anyone who has ever wandered too far to a beehive can attest, honeybees have no such apprehension about stinging humans in large swarms. Remember, honeybees in a given colony share a great deal of genetic material, so even if a few bees die, the overall gene pool survives.

Hamilton's Rule

W.D. Hamilton argued that we can quantify this behavior. According to Hamilton's rule, as long as the reproductive benefit of the act performed multiplied with the degree of genetic relatedness exceeds the reproductive cost to the individual, the individual will engage in the self-sacrificing behavior.

Now that's a lot of variables that require numbers, and we'll put some numbers to it all soon. However, the rough idea is that if an action allows for a greater chance of the collective gene pool to be passed to a following generation than had the action not occurred, then the individual will perform the action.

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