Audible endogenous motoric activity

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D. Kimbrough Oller et al., "Infant boys are more vocal than infant girls", Current Biology May 2020:

Female humans appear to have an advantage in language, from early childhood through late adulthood, reported to include a larger vocabulary, more complex utterances, greater expressive language, and better verbal and pragmatic language comprehension [1]. Wakeful infants produce ‘protophones’ — precursors to speech that include vowel-like sounds, squeals, and growls — at a rate of four or five utterances per minute, more than five times the rate of crying, throughout the first year [2]. The massive number of protophones is in itself surprising, but equally surprising, given the presumed female language advantage, we found that, in the first year, boys produced 24% more protophones than girls. This sex bias was true of infants either at high risk (HR) or low risk (LR) for autism. Both genetic and cultural factors may be involved in this bias, and additional research is clearly called for to investigate the origins of the strong tendency of infants to produce protophones and the unexpected tendency for boys to do so to a greater extent.

The difference was not only "significant" (in the statistical sense) but also meaningful — Cohen's d=0.89. Among adults, the effect size for male talkativeness is just 0.13 ("Gabby guys — the effect size", 9/23/2006).

The authors consider a range of possible explanations for the difference they found:

We wondered if the higher protophone rate of the boys would correspond to more rapid development of advanced protophones, namely canonical babbling — baba, mama, and so on — which begins at approximately seven months and involves well-formed syllables that can be used in words [4]. The canonical babbling ratio (CBR) is the number of canonical syllables, such as [ba], divided by the total number of syllables an infant produces, including non-canonical syllables, usually vowellike sounds. […]

Indeed, boys had no advantage over girls in CBR (Figure S1 in the Supplemental Information), which increased as expected significantly for both sexes across Age (p < 0.005) and Risk (LR higher, p < 0.05). Thus canonical babbling, a scaffold for first word acquisition, showed no sex bias, but did show the expected increase with age as well as a higher CBR in LR infants, a finding consistent with prior reports of disruption in canonical babbling of infants with or at risk for autism [6].  […]

It is possible that the sex difference is not closely related to language capability — the CBR did not show a sex difference — but rather to a difference in the tendency to vocalize, perhaps owing to sex differences in motoric activity level in infancy [7]. Boys might be said to show higher quantity but not quality in protophone production.

Inserting the obligatory gender stereotype at this point, a friend commented

I told my husband about this result, namely that boys just talk more, but not with more advanced sounds and he was like, "duh, and that's what they continue to do until the day they die"…lol

Continuing with Oller et al.:

This audible endogenous motoric activity, usually produced by infants in comfort, might be motivated by its value as a fitness signal for the altricial human infant, competing for parental investment [9]. One might then suggest that evolution has led to boys signaling their fitness more frequently than girls because they are more vulnerable to death in the first year [10].

Alas, "audible endogenous motoric activity" does remain common into adulthood, though I'm not convinced that there are meaningful gender differences in its frequency.


  1. bks said,

    May 26, 2020 @ 9:24 am


  2. Gregory Kusnick said,

    May 26, 2020 @ 1:51 pm

    For some reason, "audible endogenous motoric activity" summons up an image of babies cracking their knuckles.

    I also can't help wondering what sort of exogenous motor activity (audible or otherwise) the authors imagine babies are subject to.

  3. Jon said,

    May 26, 2020 @ 11:39 pm

    What's missing from every sentence is "ON AVERAGE". I expect that, as with almost every trait, the variation within one sex is much larger than the difference between the averages of each sex.

    Men are, on average, taller than women, but that fact tells you nothing about how tall any individual is. People understand that about something as obvious as height, but often lose sight of it when discussing other traits, leading to generalizations and pointless arguments.

    [(myl) See "Mandatory treatment for generic plurals" (9/13/2009) among many other posts on this topic…]

  4. eub said,

    May 27, 2020 @ 1:43 am

    Jon, can we also add "d=0.89" to each sentence?

  5. milu said,

    May 27, 2020 @ 3:54 am

    @Jon, you could copy-paste "on average" all over the place and that still would not stop unscrupulous reporters from trumpeting stuff like, "Science: Boys Make More Sound But Less Sense" or something. Sadly this kind of research is almost guaranteed to be misrepresented.

    @myl: yay sci-hub <3 but the article cuts off after the 1st page for some reason? not that you can do anything about it I guess.

    [(myl) See here.]

  6. Philip Taylor said,

    May 27, 2020 @ 4:23 am

    Milu — there are, in fact, only two pages. I can e-mail you the PDF if you are interested.

  7. milu said,

    May 27, 2020 @ 5:30 am

    @Philip— i am interested, thank you. You can send it to: miluyali at d͡ʒi:meɪl dot com. (sorry, spambots)
    Thank you!

  8. Jon said,

    May 27, 2020 @ 1:23 pm

    Milu, I'm sure you're right that it is bound to be misrepresented. But I think it is very wrong for a professional to write something like "Female humans appear to have an advantage in language" without qualification. It is naturally interpreted as referring to all female humans, compared with all males. Which is flatly untrue.

    Cohen's d is fine for statisticians. But what would be more helpful for most people would be showing the overlapping distributions of female and male results, and pointing out that most results fell within the range of overlap.

  9. milu said,

    May 27, 2020 @ 4:15 pm

    @Jon I mean I really agree, these sorts of caveats and precautions ca

  10. milu said,

    May 27, 2020 @ 4:25 pm

    …can't really be used enough, I was going to say. But this being a biology journal i would very much hope— though i don't have the field experience to say whether that is the case— that the intended audience has enough of an acquaintance with statistical analysis not to need the constant reminder that the data holds true over populations grouped according to some established criterion, etc.

    Not to mention the further limitations that this is but one study, over a small sample, and in this case (as the paper points out) from a homogenous cultural background… etc.

  11. Jon said,

    May 27, 2020 @ 5:16 pm

    Milu, I expect you are right that the intended audience will not – or at least, should not – need the reminder. My experience with professional statisticians I have known is that they don't leave such things to chance, and make sure that the necessary caveats are always present. They think that it is good practice.

    Maybe I'm a dreamer, but I would like everyone reporting results to be absolutely clear about what they are reporting.

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