The faces of elderly, happily-married people sometimes resemble each other. Dr. Aiken studied a number of couples who had been married for at least twenty-five years. Each couple provided four photographs—one photo of each partner at the time of their marriage and another photo of each partner twenty-five or more years later. All background was cut from the photos to remove any clues. The photos were then displayed in groups; a random grouping of the persons at the time of their marriage and another random grouping of the same persons who took photographs later. Some testees were asked to pick out the partners. They failed totally with the first group. Their judgments were no better than chance. But with the photos taken twenty-five or more years after the marriage, the testees were quite successful in deciding who was married to whom. They were particularly successful with the most happily-married couples. Dr. Aiken believes there are several reasons why couples grow alike. One reason has something to do with imitation. One person tends to copy or do the same as someone else without knowing it. He says human begins to imitate the expressions of the faces of their loved ones. Another possible reason, he says, is the common experience of the couples. There is a tendency for people who have the same life experience to change their faces in similar ways. For example, if a couple suffered from a lot of sad experiences, their faces are likely to change in a similar way. The main purpose of the passage is to ______.