Left-handed

The dexterity and motor ability of the left side of the body, including the left hand, is greater than that of the right side. In newborns, there is no difference in motor ability between the left and right sides because the central nervous system is immature, but at 2 to 3 years of age, individualization gradually occurs, and the dexterity of one side becomes dominant, and handedness is established. In rare cases, motor ability and dexterity on both sides are equal, which is called ambidexterity. Handedness is thought to be caused by the motor area of ​​one side of the cerebral hemisphere being more developed than the other, which results in the dexterity of the opposite limb controlled by this. It is estimated that about 65% of people have a dominant left hemisphere motor area, which explains the majority of right-handed people, but it is not clear why the motor area of ​​one side is particularly developed. Left-handed and right-handed people were also observed in experiments using mice, but there was no difference in the ratio.

However, handedness can change depending on culture. In Japan and other East Asian countries, there is a strong tradition of respecting the right side, so there is strong social pressure to correct left-handedness, and there are relatively few left-handed people. However, in modern times, for example in baseball, left-handed batting is seen as an advantage, and the number of left-handed batters who have trained to hit is rapidly increasing. In this way, handedness can change not only through natural talent, but also through experience and learning.

[Tamotsu Fujinaga]

"The Science of Right-Handedness, by Maehara Katsuya (1989, Kodansha)" ▽ "Left-Handedness is Full of Dangers, by Stanley Cohn, translated by Ishiyama Suzuko (1994, Bungeishunju)" ▽ "Do-It-Yourself Psychology -- Version Up, edited by Miyazawa Hidetsugu (1997, Nakanishiya Publishing)" ▽ "Interesting Stories about Right and Left, edited by Surprise Data Information Department (1998, Kawade Shobo Shinsha)" ▽ "The Invisible Left Hand -- Proposals for a Silent Social System, by Oji Naoya (1998, Sangokan)" ▽ "Felissimo Left-Handed Friendship Association, edited by Oji Naoya, Let's Be Left-Handed! -- For the Loveable Individuality of the 21st Century (2000, Felissimo Publishing)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

左手をはじめとする身体左側の器用さや運動能力が、右側のそれを上回ること。新生児では中枢神経系の未成熟のため左右の運動能に差はないが、2～3歳でしだいに個性化してどちらかの側の器用さが優位となり、利き手が確立する。まれに両側の運動能や器用さが対等の場合もあるが、これは両手利きとよばれる。利き手の原因は、どちらかの側の大脳半球の運動野が他側よりよく発達しているためと考えられ、これによって支配されている反対側の手足の器用さが勝ることとなる。左半球の運動野が優位な人は約65％と見積もられ、これが右利きの多数を説明するが、なぜ一側の運動野がとくに発達するかは明らかでない。ネズミによる実験でも左利きと右利きが認められたが、その比率には差がなかった。

　しかし、利き手は、文化によっても優位性が変化する。日本をはじめ東アジア諸国では右側尊重の伝統が強いので、左利き矯正(きょうせい)への社会的圧力が高く、左利きは相対的に少なかったが、現代では野球を例にとると、左打ちが有利とみられるため、訓練による左打ちの打者が急増している。このように素質だけではなく、経験や学習によっても利き側は変わる。

［藤永　保］

『前原勝矢著『右利き・左利きの科学』（1989・講談社）』▽『スタンレー・コーン著、石山鈴子訳『左利きは危険がいっぱい』（1994・文芸春秋）』▽『宮沢秀次編著『自分でできる心理学――ばーじょんあっぷ』（1997・ナカニシヤ出版）』▽『びっくりデータ情報部編『「右」と「左」の気になる面白話』（1998・河出書房新社）』▽『大路直哉著『見えざる左手――ものいわぬ社会制度への提言』（1998・三五館）』▽『フェリシモ左きき友の会・大路直哉編著『左ききでいこう！――愛すべき21世紀の個性のために』（2000・フェリシモ出版）』

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