Part 4: Left-Handedness
You will hear part of a lecture on left-handedness.
Lời thoại bài nghe (transcript)
Lecturer: Good morning. In today's lecture in our series on human variation, I'm going to look at a trait that most of us take for granted: which hand we prefer to use. Roughly one person in ten is left-handed, and what's remarkable is how stable that figure seems to be. It's similar in every region where it has been measured, and evidence from the past suggests it hasn't changed much over thousands of years. Archaeologists, for example, have examined the marks left on ancient stone tools, and the angle of the strikes indicates that most of the people who made them were right-handed, in roughly the same proportion as today.
Lecturer: So what makes a person left-handed? For a long time, people assumed there must be a single gene responsible, a 'left-handed gene' that was passed down in families. That idea has now been abandoned. Handedness does run in families to some extent, but large genetic studies suggest that dozens of genes are involved, each having only a very small effect. Strong evidence that genes are not the whole story comes from twins. Identical twins share all their DNA, and yet in roughly one pair in five, one twin is left-handed and the other right-handed.
Lecturer: Interestingly, hand preference seems to develop very early, before birth, in fact. Ultrasound scans have shown that most babies in the womb suck their right thumb more often than their left, and follow-up studies found that this early preference was a fairly good predictor of which hand the child later used. This suggests that conditions in the womb play a part, although exactly how is still unclear.
Lecturer: Let's turn now to social factors. For much of history, left-handedness was regarded with suspicion, and in many countries children were made to use their right hand, particularly for writing. Teachers would sometimes even tie a child's left hand behind their back. In countries where this pressure gradually disappeared during the twentieth century, the proportion of people who reported being left-handed rose quite sharply. That wasn't because more left-handers were being born, but because fewer were being forced to switch. In some parts of the world this kind of pressure still exists, which is why surveys there record much lower figures.
Lecturer: Even where they're no longer made to change, left-handers live in a world designed largely for right-handers. Everyday objects such as scissors, tin openers and computer mice are usually shaped for the right hand, and many people don't realise how awkward they are until they watch a left-hander struggle with them. Writing can also be a problem. Because a left-hander's hand moves across the words they have just written, they tend to smudge the ink, which is why many adopt an unusual hooked position.
Lecturer: On the other hand, if you'll excuse the pun, left-handedness can bring advantages. In sports where players face an opponent directly, such as tennis, boxing and fencing, left-handers are found at the top level far more often than you'd expect. The usual explanation is that most players spend their time competing against right-handers, so they're less prepared for a left-handed opponent. Crucially, there's no such advantage in sports where competitors don't interact directly, like swimming or gymnastics, where left-handers appear in roughly the normal proportion.
Lecturer: This has led some researchers to suggest that something similar may explain why left-handedness has survived at all. In our evolutionary past, being left-handed may have given people an advantage in fighting, for the same reason it helps in tennis today. But the advantage only works as long as left-handers remain rare, which might explain why they've stayed at around ten per cent rather than becoming more common. It's an elegant theory, though I should stress that it's very difficult to test directly. Next week we'll move on to look at colour vision.
