That's one theory. Another one I can think of is that sharp edges are scary, and most distress calls are high pitched.

Also, the thing about high frequencies and sharp edges lead to a contradiction: babies are more round than adults and produce higher pitched sounds, this is almost universal across all species.

There are other tentative explanations, such as how the vocal tract acts when producing these sounds, with "bouba" sounds being the result of smoother movement more reminiscent of a round shape.

"kiki" is not just higher pitched, it is also "shaped" differently if you look at the sound envelope, with, as expected, sharper transitions.

So to me, the mystery is still there. Is is the kind of thing that sounds obvious, in the same way that kiki sounds obviously sharper than bouba, but is not.

In nature there's few things laying around that resonate particularly well.

I thought the same but they used chicks that just hatched with zero world experience.

So then the mystery would be why other primates do not appear to show the bouba-kiki effect ...

> The "kiki" sound has more high frequency content than the "bouba" sound

And where did you get that from? In non-tonal languages the pitch conveys almost no information and people speak at very different ones (and for instance a male saying "kiki" will say it at lower frequencies than a woman saying "bouba" most of the time) so I find your affirmation very dubious.

> and it's no mystery why the brain associates one with the other.

Specialists of the field find that mysterious but some smartass on HN disagrees.

That's what I was thinking. But then I was wondering: if it was that obvoius, would there be such research about it?