53.Thirteen years ago, researchers studied a group of 25 infants who showed signs of mild distress when exposed to unfamiliar stimuli such as an unusual odor or a tape recording of an unknown voice. They discovered that these infants were more likely than other infants to have been conceived in early autumn, a time when their mothers' production of melatonin—a hormone known to affect some brain functions—would naturally increase in response to decreased daylight. In a follow-up study conducted earlier this year, more than half of these children—now teenagers—who had shown signs of distress identified themselves as shy. Clearly, increased levels of melatonin before birth cause shyness during infancy and this shyness continues into later life.(523)
In this argument, the author claims that the increase of mothers' melatonin is the reason that they give birth to shy babies which still exist when babies grow up. To support this claim, the author cites a study that shy babies are more likely to be conceived in a time when their mothers' melatonin is increasing. He also provides a follow-up study of these shy children when they are teenagers. Close examination of these supporting evidence, however, reveals that the argument suffers from a series of unproven assumptions which render it unconvincing as it stands.
A threshold problem with the argument involves the statistical reliability of the study. The author provides no evidence that the number of the infants is statistically significant or that the infants are representative of the entire population of the infants that are sensitive to unfamiliar stimuli. Perhaps just most of the 25 infants happen to be conceived in early autumn, while most shy babies in general are conceived in early spring. In short, lacking evidence of a sufficiently representative sample, the author cannot justifiable rely on the study to draw any conclusion.
Even if the study's sample is representative of the overall population of shy infants, the argument relies on the assumption that in the follow-up study when they are teenagers their responses are reliable. Yet the author ignores the possibility that these children are too young to identify themselves well and truly. Lacking evidence that the children's reports are both truthful and meaningful, the author cannot make a convincing conclusion based on them.
Nor can the author justify the exclusive causal relationship between the increase of mothers' melatonin and the shyness of their children on the basis of the coincidence of these two facts. Perhaps it is the increased levels of another hormone rather than melatonin before birth causing shyness during infancy, or perhaps increase of melatonin is not the only reason for shy infants and other reasons such as existence of certain gene also result in the shyness. Without considering and ruling out these and other possibilities for shyness, the author cannot confidently conclude that infants' shyness only attribute to increased levels of mothers' melatonin.
Even assuming that the study is credible and the shyness results from the increasing melatonin, it is nevertheless impossible to assess the author's further conclusion that the shyness will continue into later life. Only more than half of these children who all showed signs of distress during infancy are shy when they are teenagers. Perhaps when they continue to grow up, much fewer would be shy. For that matter, this shyness is gradually disappearing during their later life. Therefore, without more information of these children's situation when they grow older, the author's conclusion is simply unjustified.
To sum up, the conclusion lacks credibility because the evidence cited in the argument does not lend strong support to what the author maintains. To strengthen the argument, the author would have to provide more evidence concerning the relationship between mothers' melatonin and infants' shyness. To better evaluate the argument, more information about children's situation in the next few years would also be very useful.
发表于 2007-6-23 16:38:02
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