user-1: 
What is the evolutionary explanation for why lions sometimes kill their own cubs when they acquire a new pride?

Is definitely on-topic to Biology.SE

What is the evolutionary explanation for why people pick their nails as a stress-reliever?

Would be considered sort of in the middle between Biology.SE and Psychology.SE

What is the evolutionary explanation for domestic violence being so common despite the fact that it compromises the welfare of offspring?

Would be considered "off-topic" to Biology.SE since it discusses human psychology.
Another question asked about the gray area between biology and psychology but this question is specific to evolutionary psychology, since evolution is notoriously a "biology" subject, I imagine a lot of questions that discuss "what is the evolutionary purpose (explanation) of (random psychological trait or habit)" get closed for mixing up the two, but 'evolutionary explanation' does throw a wrench in the equation, I think, because maybe the asker would get a better explanation about evolution here than over there, even if it is in the context of humans (after all, humans evolved similarly to other animals). What are the defined barriers?

user22057: Any question starting:

What is the evolutionary explanation for…

should not be allowed on SE Biology (at least until it has been reworded) because it contains the implicit assumption that the phenomenon in question has arisen through evolutionary pressure. In many cases this is manifestly ridiculous as it relates to recent human behaviour. But on a scientific forum like this such questions should be of the form:

Is there an evolutionary explanation for…

Often questioners are really asking:

What purpose is served by…

in which case the questions should be simplified to say that.
In my opinion we are too reluctant to edit questions for the legitimate purpose of improving them. This both raises the standards of the site and, one hopes, educates posters to consider the assumptions in their questions and whether they are valid.

user27148: There are certainly overlaps where questions are on-topic between multiple SE sites. It's unfortunate that the SE software does not handle that well by allowing cross-site duplicates, etc, but that's another story.
However, most "what's the evolutionary explanation for _______" questions asked here get closed as a duplicate of this one:
Why do some bad traits evolve, and good ones don't?
When people ask them on the other sites, I wish they'd ask them here so we could also close as a duplicate of this one. It has several distinct answers that address the issue from multiple perspectives. I'd highly recommend looking carefully at it before asking any of these sorts of questions on any SE site.
Evolutionary psychology in particular has received a lot of criticism for putting out theories that are not testable by science and are rather "just-so" stories. Then, you have people that confuse this with a weakness or criticism of evolutionary biology more broadly, when really that's just not how evolutionary biology works.

-1: <blockquote>
<p>What is the evolutionary explanation for why lions sometimes kill their own cubs when they acquire a new pride?</p>
</blockquote>
<p>Is definitely on-topic to Biology.SE</p>
<blockquote>
<p>What is the evolutionary explanation for why people pick their nails as a stress-reliever?</p>
</blockquote>
<p>Would be considered sort of in the middle between Biology.SE and Psychology.SE</p>
<blockquote>
<p>What is the evolutionary explanation for domestic violence being so common despite the fact that it compromises the welfare of offspring?</p>
</blockquote>
<p>Would be considered &quot;off-topic&quot; to Biology.SE since it discusses human psychology.</p>
<p>Another question asked about the gray area between biology and psychology but this question is specific to evolutionary psychology, since evolution is notoriously a &quot;biology&quot; subject, I imagine a lot of questions that discuss &quot;what is the evolutionary <em>purpose</em> (explanation) of (random psychological trait or habit)&quot; get closed for mixing up the two, but 'evolutionary explanation' does throw a wrench in the equation, I think, because maybe the asker would get a better explanation about evolution here than over there, even if it is in the context of humans (after all, humans evolved similarly to other animals). What are the defined barriers?</p>
 22057: <p>Any question starting:</p>
<blockquote>
<p>What is the evolutionary explanation for…</p>
</blockquote>
<p>should not be allowed on SE Biology (at least until it has been reworded) because it contains the implicit assumption that the phenomenon in question has arisen through evolutionary pressure. In many cases this is manifestly ridiculous as it relates to recent human behaviour. But on a scientific forum like this such questions should be of the form:</p>
<blockquote>
<p>Is there an evolutionary explanation for…</p>
</blockquote>
<p>Often questioners are really asking:</p>
<blockquote>
<p>What purpose is served by…</p>
</blockquote>
<p>in which case the questions should be simplified to say that.</p>
<p>In my opinion we are too reluctant to edit questions for the legitimate purpose of improving them. This both raises the standards of the site and, one hopes, educates posters to consider the assumptions in their questions and whether they are valid.</p>
 27148: <p>There are certainly overlaps where questions are on-topic between multiple SE sites. It's unfortunate that the SE software does not handle that well by allowing cross-site duplicates, etc, but that's another story.</p>
<p>However, most &quot;what's the evolutionary explanation for _______&quot; questions asked here get closed as a duplicate of this one:</p>
<p><a href="https://biology.stackexchange.com/questions/35532/why-do-some-bad-traits-evolve-and-good-ones-dont">Why do some bad traits evolve, and good ones don&#39;t?</a></p>
<p>When people ask them on the other sites, I wish they'd ask them here so we could also close as a duplicate of this one. It has several distinct answers that address the issue from multiple perspectives. I'd highly recommend looking carefully at it before asking any of these sorts of questions on any SE site.</p>
<p>Evolutionary psychology in particular has received a lot of criticism for putting out theories that are not testable by science and are rather &quot;just-so&quot; stories. Then, you have people that confuse this with a weakness or criticism of evolutionary biology more broadly, when really that's just not how evolutionary biology works.</p>
