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I remember reading about a concept—in evolutionary biology or natural selection, I think—whereby a particular trait is advantageous to the population or species but only so long as that trait is only exhibited by a minority of the population. That is, the population is more likely to survive if the trait's existence and expression is maintained, but only if that expression is limited to a small percentage of the population. If the trait becomes too common, the selection advantage of having it will decrease, and the fact that most of the population has it may even negatively impact the survivability of that population.

I can't recall where I encountered it. It may have been in the context of reading about ADD, Aspberger's/Autism, neurotypicality, introversion, or mania/OCD. I want to revisit the topic and learn some more about it, but Google is failing me.

Can anyone tell me the name of or term for the concept I'm thinking of?

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  • $\begingroup$ If I recall correctly, in humans, there is a recessive genetic trait which gives resistance to malaria in the heterozygous state (Tt) but causes malformed red blood cells in the homozygous recessive state (tt). Surely, this qualifies as an example, because if the gene is uncommon, the homozygous recessive becomes rare? $\endgroup$
    – nickalh
    May 22, 2015 at 20:27
  • $\begingroup$ @nickalh I don't think that counts as an example. I think the term "heterozygote advantage" more accurately describes the behavior of the sickle-cell condition. $\endgroup$
    – C_Z_
    May 22, 2015 at 22:42

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Frequency-dependent selection is the term you are looking for, I believe. Positive frequency-dependent selection encompasses traits that become more advantageous as they become more common. Negative frequency-dependent selection encompasses traits that become more advantageous as they become rarer.

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