问答题 Forget expensive educational DVDs and private tutors, the secret to smart children could be as simple as giving birth to them two years apart. Researchers who studied thousands of children found a two-year gap to be optimum in boosting brain power. Any shorter, and the reading and maths skills of the older child dipped. The effect was strongest between the first and second-born, but siblings in bigger families also benefited. The theory comes from Kasey Buckles, an economist whose own children are, rather fortunately, just over two years apart in age. She said it is likely that the difference in academic achievement is linked to the time and resources parents can invest in a child before a younger sibling arrives. However, waiting more than two years did not increase the advantage, the Journal of Human Resources will report. 无