This might seem a bit weird, but why can only women produce milk? Wouldn't it be more beneficial to our species if all humans could produce it? I can't think of any reason only one gender would need it, unless it would somehow be harmful to men.

I know that men can secrete milk in some cases, but why not in general?

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    $\begingroup$ The words beneficial to our species very much suggests that you misunderstand evolutionary processes. $\endgroup$ – Remi.b Dec 28 '15 at 18:40
  • $\begingroup$ Why do you say that, Remi? Traits that are detrimental tend to not be selected for in many ways, while traits that are beneficial increase the likelihood that an organism will pass on those traits to future generations. That is the theory of natural selection in a nutshell. $\endgroup$ – DonJulian Dec 28 '15 at 18:49
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    $\begingroup$ Evolution does not primarily work for the benefit of the species @DonJulian $\endgroup$ – rg255 Dec 28 '15 at 19:16
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    $\begingroup$ related/possible duplicate: Why only female mammals have well developed mammary glands? $\endgroup$ – fileunderwater Dec 28 '15 at 19:40