We know that babies pick up on the "affect" or emotional content of speech rather than the actual words, says Gerald McRoberts, a psychologist at Lehigh University in Bethlehem, Pennsylvania. But it is still unclear precisely how adults use various acoustic properties in their voice, such as rhythm, pitch and stress, to communicate different meanings to infants.
So McRoberts, and Malcolm Slaney of IBM's Almaden Research Center in San Jose, California, designed a computer program to evaluate the properties of the speech parents use as they talk to their children. They then asked six sets of parents to play with their infants and make approving or disapproving comments designed either to encourage the child, or to warn them to stay away from dangerous objects such as sharp instruments or electrical appliances.
When the program analysed the acoustic properties of nearly 700 excerpts of speech, it correctly distinguished between approving or disapproving comments 80 per cent of the time. But to the researchers' surprise, the program correctly identified 12 per cent more of the comments made by the mothers, suggesting that women use less ambiguous sounds than men to convey to babies what they mean (Speech Communications, vol 39, p 367).
McRoberts admits it is possible that men communicate as effectively as women, but do so using speech characteristics that the program failed to pick up. Or they may have been less relaxed in the lab. But the experiment does show unequivocally that the mothers and fathers talked to children differently.
The only way to be sure whether babies can tell the difference would be to "ask" them- for instance, by seeing how they respond to voices as their acoustic parameters are systematically changed. This would be difficult for many reasons, says McRoberts. "Not the least of which is that you can make a baby smile or laugh lots of times in an experiment but you can only make them cry once- then it's all over."
New Scientist issue: 5th February 2002
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