Computers can now beat the best human minds in the most intellectually challenging games like chess. They can also__26__tasks that are difficult for adult humans to learn, such as driving cars. Yet autonomous machines have difficulty learning to co-operate. That’s something__27__young children do. Human co-operation__28__easy, but it’s very difficult to imitate because it__29__cultural norms, deeply rooted instincts, and social mechanisms that express disapproval of non -collaborative behavior. Such common sense mechanisms aren’t easily built into machines. Thus, the same AI software programs that effectively play the board games of chess and checkers, Atari video games, and the card game of poker, often__30__to consistently co-operate when it’s necessary.