If your point is that going on about evolutionary psychology adds to the obfuscation but not to the insensitivity, I disagree.

It could go either way. Digression into a bunch of theory and science impersonalizes things as well as focussing on 'me' instead of 'you' The main problem with getting into a big speel on science is that it increases the total time spent dwelling on the painfully negative topic. The fact that it is talking about the science isn't the insulting part.

There are often ways of more or less sensitively coming clean about (what one takes to be) one's true reasons for breaking up. Maybe you wouldn't go so specific as "you're too fat," but you could talk about lack of physical chemistry or whatever without uttering a falsehood or being too misunderstood.

Talking about 'lack of physical chemistry' is less insulting by virtue of being a vague pre-packaged euphemism rather than brutally personal criticism of highly status-sensitive personal features. It seems to be an entirely different kind of difference to whether you mention evolutionary psychology or not.

I think you're assuming too rational a partner.

If you're honest and say, "Your breasts are too small," the person in question might seek a guy who likes smaller breasts next time. Or she might fall into a deep self-loathing in which she believes that her body is imperfect and nobody could be attracted to her, thus sabotaging her own future potential relationships. Or she could run out and get breast implants, even though she doesn't really want them, in hopes that you / other future guys will find her more attractive--which is much more expensive and possibly less rewarding than simply finding people who like small breasts.

In my view it's better to keep it vague. Guessing over dozens of possible flaws is likely to be less harmful than obsessing over one particular flaw, since it's difficult to figure out / change whatever possible flaw you think may exist.

(Disclosure: I have been dumped once and did the dumping once. The dumper kept it vague; I kept it specific but lied. I can't judge how keeping it specific while lying worked, since the person in question was bipolar and therefore not at all a normal test subject. I can judge how keeping it vague went: I obsessed over dozens of flaws for a while, until I found other people who were interested, at which point I decided it was probably just a bad match and nothing really to do with absolute flaws at all. I do not know how a completely honest dumping pans out.)

true reason for rejecting them

This usually makes little sense, particularly for someone one was attracted to for a while.

It's almost never true that for someone whose breasts one once found sufficient, her breasts would be a deal breaker, and no woman would be attractive with similar breasts regardless of her personality, face, legs, etc.

The problem is that the character sheet was filled out with mostly low die rolls, not that stat X is too low.

ETA: asking what the "true reason" for a breakup was is like asking what the "true reason" for a war, such as the Iraq War, was. Was it possible WMD? Past links to Al-Qaida? Possible future links to Al-Qaida? Past human rights abuses such as mass torture and murder? Aquiring influence over oil? Creating a pro-western regime? Creating a democratic regime? Perceived divine guidance during Bush's praying?

The first test to figure out if someone is more rationalist than emotional about the Iraq war to ask them what the "true reason" for the invasion was and see if they right that wrong question. It's just as much the wrong question in this context as that one.

Calling it "grossly insensitive" is too quick a rejection of the possibility of telling a truth.

I agree.

It would depend on the broader social context - in particular, will you still share a social context - but if you do it seems likely you could get your name dragged through the mud in that example,and she still might not believe you and so suffer b) anyway.

A complementary position is that just because something is 'grossly insensitive' doesn't mean it isn't both a kindness and exactly the right thing to do. Humans learn from unpleasant things. Especially targeted unpleasant things. So 'got to be cruel to be kind' often applies.

If your true and actual reason for breaking up with someone is that her breasts are too small, consider that (a) saying "It was because you were too clingy" may cause them to try and mess with an aspect of their personality that doesn't even need fixing, and (b) total silence, which you may fondly imagine to be mercy, may result in her frantically imagining dozens of possible flaws all of which she tries desperately to correct, just on the off-chance it was that one. As opposed to, say, looking for a guy who's into smaller breasts next time.

Tangent: The tricky thing is that often "because you were too clingy" will technically be the real reason, just not the most useful part of the causal chain to select. If she had bigger breasts that will change both how 'clingy' any given behavior seems and how much attraction to her you exhibit which in turn influences how clingy she is likely to be. So sometimes even 'real' reasons can be a cop-out!

Maybe I'm just being inordinately naive, but telling someone honestly, softly, and believably, your true reason for rejecting them, seems like it really should have certain advantages for them, if not for you.

That certainly seems likely for most cases.

Even bigger tangent: I can't think of many better ways to be broken up with than this! Seriously. It's (counter-intuitively) one of the least personally insulting break ups I've seen. Because pussy-footying around being 'sensitive' is in its own way just another kind of insult.

WOW! I'd call this the most credible surprising argument for truthfulness I have seen in a long time. Figures it's from Eliezer. Score in our years long argument over the strength of the prior for truthfulness.
Note though, that to be a good idea this would have to be done very sensitively. Also, the girl would have to be awfully rationalistic. I'd default to the position that any girl who isn't already poly is fairly unlikely to be a good candidate for this sort of argument, accompanied by a firm assertion that rationalist guys should not restrict themselves to poly girls.

On the other hand, I don't think the author was collecting cross cultural material, so I don't know what insults/shame looks like in cultures where religious prejudice is a larger factor.

I'm not an expert on this by any means, but insult content varies quite a bit across cultures. Dutch profanity is largely medical, for example; kankerlijer ("cancer-sufferer") is very strong. I've heard speculation that this is due to the largely urban landscape that Dutch evolved in, where being a vector of infection (never mind that most causes of cancer aren't infectious) meant being a clear danger to the community.

So religious insults seem plausible in any culture that takes religion especially seriously. Spanish profanity has retained a lot of religious content; I don't know much about its evolution, but it could plausibly be related to Spanish-speaking culture's historically strong Catholicism.

It's worth noting that not every piece of social interaction has a non-trivial influence from evolutionary psychology. Sometimes an insult is just an insult...

Humans have emotions and don't think rationally by default. Most people do not like to feel inadequate, though how they respond to that feeling varies a great deal. Most people in a relationship also don't like to feel they were rejected sexually over some perceived inadequacy.

So when a mate gives them a 20-page lecture on their failures to hold their attraction and concludes by rejecting them as a sex partner, it's probably not vanishingly far from the null hypothesis that the person is going to get upset...

Well it's almost definitional. If evolutionary selection pressures were extreme enough to actually make lukeprog that way, then all men are that way. If evolution did it to him, then it did it to everyone. Evolution doesn't discriminate. What's more likely is that evolution didn't actually make him that way, but societal pressures did.

But that's setting aside the fact that most people tend to wildly anthropomorphize evolution...