乳児期の男児は女児よりもおしゃべり　米研究

【ワシントンＡＦＰ＝時事】生後１年までの男児は女児よりもおしゃべりだとする研究論文が５月３１日、学術誌「アイサイエンス」に発表された。女性の方が幼い頃から言語能力が高いとする通説を覆す結果に研究チームも驚いている。（写真はフランス・パリの病院内で遊ぶ子どもたち〈資料写真〉）
　米メンフィス大学のキンブロー・オレー博士らのチームは、２年間で乳幼児５８９９人の音声を計４５万時間以上録音し、アルゴリズムを使って精査した。言語発達に関する研究サンプルとしては過去最大規模だという。
　言葉をまだ話すことができない乳幼児も、最初に鳴き声やうなり声、唇を使った音、その後「バババ」「ガガガ」のような「喃語（なんご）」と呼ばれる連続音などを発する。
　言語習得は男児より女児の方が早いという考えは長年、科学界で支持され、前言語期のこうした音も女児の方がよく発すると考えられてきた。
　だが今回の研究では、生後１年の段階で男児の方が女児よりも１０％多く音を発していた。２年目になると女児が追いつき、男児よりも７％多く音を発していた。
　１年目も２年目も周囲の大人は男児よりも女児に多く話しかけていたにもかかわらず、こうした結果が出たという。
　■進化論
　研究チームはこの結果について、乳幼児が発する音は養育者に自分の健康状態を知らせるためだとする進化論の一説に合致する可能性があるとしている。
　これまでの多くの研究により、生後１年目の死亡率は男児の方が女児よりも高いことが明らかになっている。遠い昔には、より音を発する男児が生き残り、遺伝子を残す確率が高かったと言えるかもしれない。
　オラー氏のチームは、乳幼児の発する音への養育者の反応について研究を進めるとしている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2023/06/01-17:15）

Baby boys babble more than girls, according to a scientific paper out Wednesday that upends a common belief that females hold a language advantage over males early on in life.
The findings, published in iScience after the largest ever study on the subject, came as a surprise even to the paper's authors.
They say it might be the result of an important sex difference that emerged during our species' evolution.
A team led by D. Kimbrough Oller of the University of Memphis, Tennessee used an algorithm to trawl through a data set of more than 450,000 hours of non-stop audio from 5,899 infants, recorded using an iPod sized device over two years.
This is the biggest sample for any study ever conducted on language development, as far as we know, Oller said in a statement.
While young babies don't talk, they produce pre-speech vocalizations -- squeals, growls, raspberries, and later word-like sounds such as ba and ga -- collectively called protophones that eventually give way to real words and sentences.
The idea that girls acquire language faster than boys has long held sway in scientific circles, and with it the assumption that baby girls vocalize more than baby boys.
However, the results showed that boys made 10 percent more utterances in the first year of life, before the girls caught up and made seven percent more sounds by the second year.
- Evolutionary theory -
The differences occurred despite the fact that adult care-givers spoke more to girls than to boys across both years.
One theory for the finding was that male infants were more vocal because they were more active in general. But the data did not support this, since higher male vocalizations gave way to females around the 16 month mark, but higher physical activity did not.
Instead, the team suggests their findings might fit an evolutionary theory which holds that infants make sounds in order to signal their wellbeing to their caregivers, who in turn invest more energy and attention in them.
Boys have higher mortality rates than girls in their first year of life, according to a broad body of research, and so it may follow that more vocal baby boys in the distant past were more likely to survive and pass on their genes.
But by the second year of life, death rates have dropped dramatically for both sexes, and the pressure on special fitness signaling is lower for both boys and girls, said Oller.
Next, Oller plans more research on how caregivers respond to baby talk.
We anticipate that caregivers will show discernible reactions of interest and of being charmed by the speech-like sounds, indicators that fitness-signaling by the baby elicits real feelings of fondness and willingness to invest in the well-being of infants, he said.