Most explanations for humor neglect important types of humor, such as tickling and word play; or raise difficult evolutionary theoretical problems, such as group selection, dubious fitness benefits, and excessive complexity of design; or ignore the data on humor and laughter. The present theory was based on the following observations. Tickling entails a mock attack at vulnerable body spots, and may provide youngsters with practice in defending themselves. The child's laughter is pleasant and encourages the tickler to persist. Similarly, juvenile primates including children encourage roughhousing by laughter and other emotional expressions. We also laugh at humorous content that provides striking counter-examples (incongruities), as in word play, or that informs us about fitness-relevant topics such as sexual, aggressive, and social poise scenarios. The present theory is that the pleasure of humor motivates us to seek out poignant, fitness- enhancing input of this sort. Laughter evolved to allow us to continue to recieve amusement. Laughter is a pleasant social signal that prompts the humorist to persist in providing this edifying stimulation. In response to true wit, laughter conveys appreciation and gratitude—an intention to reciprocate for having received a stimulating idea. Thus, humor benefits both humorist and laughter. This theory and others are evaluated in the light of evolutionary principles and relevant data.