at a cost to the punisher. Harvard researcher David Rand says the most successful behavior proved to be cooperation. The groups that rewarded it the most earned about twice as much in the game as the groups that rewarded it the least. And the more a group punished itself，

　　the lower its earnings. The group with the most punishment earned twenty­five percent less than the group with the least punishment. The study appeared last month in the journal Science.

　　The other study involved children. It was presented last month in California at a conference on violence and abuse. Researchers used intelligence tests given to two groups. More than eight hundred children were aged two to four the first time they were tested. More than seven hundred children were aged five to nine. The two groups were retested four years later，

　　and the study compared the results with the first test. Both groups contained children whose parents used physical punishment and children whose parents did not.

　　The study says the IQs of the younger children who were not spanked were five points higher than those who were. In the older group，

　　the difference was almost three points. The more they are spanked，

　　the slower their mental development is.

　　31．According to the first study，

　　we mainly infer that________.

　　A．the game is called Prisoner's Dilemma

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