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According to this article, only about 1% of all humans are ambidextrous:

About 90 percent of people are right-handed, says Corballis. The remaining 10 percent are either left-handed or some degree of ambidextrous, though people with "true" ambidexterity—i.e., no dominant hand at all—only make up about 1 percent of the population.

Less dominance is observed in other animals, as indicated by this article:

Bill Hopkins, at the Yerkes Regional Primate Research Center in Atlanta, USA, has made even more contentious claims in recent years, asserting that his captive chimpanzees exhibit a 70% right-handed preference for many tasks requiring manual dexterity, a rate that rises to almost 98% for very specific tasks, like the precise over-arm throwing of objects. However, for everyday food-related tasks (like cracking nuts or digging out honey), the rate remains closer to 50-50.

I am wondering if there is any theory that explains why ambidextrous persons are so rare. As a totally layman when it comes to biology it seems that this would require a more complex brain to wire all the complexity to both members and this would be rarely justified (virtually all tasks can be done without being fully ambidextrous).

Question: Why are ambidextrous persons so rare?

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    $\begingroup$ Speculation: Left-right hemisphere competition and dominance became more pronounced as our brains evolved to do more and more cognitive work. Hand dominance increased as a side effect and there was never any selective pressure to maintain ambidexterity. $\endgroup$
    – Armatus
    Apr 6, 2018 at 9:37
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    $\begingroup$ It would be interesting to know whether, and to what degree, it is simply a matter of training. That is, once a child displays handedness, perhaps in early writing or ball games, it is encouraged to practice doing things with the dominant hand. But from my own experience, when one temporarily loses the use of that hand (as with a broken wrist), it is possible to learn to do many things adequately well with the non-dominant hand. $\endgroup$
    – jamesqf
    Apr 6, 2018 at 20:36

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While not specifically answering why ambidextrous persons are rare, one potential explanation is that mixed-handedness and/or ambidexterity seems to be related to adverse effects. For instance, some studies have shown that mixed-handedness (which might not be exactly the same as ambidextrous, depending on how you define the terms) is related to ADHD symptoms (e.g. Rodriguez et al. 2010) and patients with schizophrenia have an overrepresentation of mixed-handedness (Green et al, 1989; Orr et al, 1999) and ambidexterity (Crow et al, 1996).

A potential reason for the adverse effects seems to be that mixed-handedness and ambidexterity is related to delayed or weakened dominance of one cerebral hemisphere, which is important for language development (see e.g. Crow et al, 1996). See also this quote from Intelligence and the ‘Intelligence Quotient’ (Crow, 2001 from Encyclopedia of Genetics):

Handedness, reflecting cerebral dominance, is a trait that is associated with quantitative variation. Whether this variation is a correlate of human cognitive ability, as would seem plausible if it underlies the specific characteristic of language, has been much debated, but it now appears that lesser degrees of lateralization (‘hemispheric indecision’) are associated with delay in the development of verbal, and also nonverbal, ability (Crow et al., 1998). Thus it appears that lateralization is associated with significant variation in the rate at which words acquire meaning, and that this variation reflects a dimension that is specific to Homo sapiens.

This is not my area of expertice thought, and others more knowledgeable about the field can maybe expand on this or explain more in detail.

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