单项选择题

Earlier this month, researcher Kevin Beaver of Florida State University reported that he and his co-authors had identified genetic markers associated with academic achievement. In their study, published in the journal Developmental Psychology, the scientists found that young people who possessed particular forms of three genes were more likely to finish high school and go on to college than those who carried other forms of the genes. The newly found genes are involved in regulating the actions in the brain, and have been linked in other studies to levels of motivation, attention and intelligence.
It is true that in recent years, scientists have produced a growing number of studies linking the capacity to learn to specific genes. A team at King’s College London, for example, has published several articles relating ability in mathematics to different varieties in DNA. Children who carried 10 or more of the "risk" genes identified by the researchers were nearly twice as likely to perform poorly in math, according to a 2010 study generated by the group. In another experiment, scientists reported that some individuals possess another two forms of genes involved in brain development that may make it easier for them to learn difficult languages like Chinese.
But scientists have long warned against attributing complex human behaviors to the action of a few genes--and learning is among the most complex things we do. The authors of these studies acknowledge this. "Mathematical ability and disability are influenced by many forms of genes generating small effects across the entire system of ability," writes Sophia Docherty, who heads the King’s College team. Moreover, environment matters, even in the context of genes: Docherty’s research finds that children with the "risk" forms of genes were especially likely to do poorly in math when they lived in chaotic homes and had negative, punitive(惩罚性的)parents. More generally research indicates that genetic factors account for about half of the elements in educational achievement.

The word "chaotic" probably means ().

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