Similarity—of attitudes, background, goals, beliefs, interests, values—has been shown to be a powerful predictor of interpersonal attraction. Robert Zajonc and his colleagues have investigated similarity and relationships from a slightly different perspective, with fascinating results.

Zajonc and his coworkers solicited photographs from couples in Michigan and Wisconsin who had lived together for 25 years. These couples were asked to supply photos taken when they were first involved with one another and more recently, after this long period of time together; many couples provided shots from their wedding and from their 25th anniversary. The photographs were cropped so that only the faces of the men and women were visible. This was done so that other cues such as clothing styles or background scenes were not visible. The shots of these men and women were then presented to groups of judges, whose task was a relatively simple one. Presented with the photo of a young woman, for example, and an array of 6 photographs of young men, participants were asked to simply match "who went with whom," or to identify the woman's spouse from the array of men. (The other combinations—male targets and female options, photos of the older partners, and so on—were also represented in the experimental design.)

As an antecedent of interpersonal attraction, the similarity thesis might predict that couples resembled one another early in their relationship, thereby drawing them together. This is not what Zajonc found, however. Raters were no better than chance at matching couples based on their young photographs. They were quite accurate, however, in matching the older photographs. At levels better than chance, participants could tell who resembled whom after 25 years of marriage. This couldn't be due to artifactual cues (scenes in the background, quality of the images) or methods of presentation, as these elements were carefully controlled. The conclusion is inescapable; these couples grew to look like one another in facial resemblance after years of marriage.

Zajonc offers some suggestions as to why this might occur. First, diet could reasonably be implicated. Couples tend to adopt the same dietary habits, so it is possible that years of eating high- or low-fat diets could lead to similar fatty deposits in the face, in turn producing greater physical resemblance. However, a rank correlation between weight and age failed to support this notion. Another possibility is that environmental factors could have played a role. Twenty-five years of living in Texas, for example, could produce the leathery skin cues that distinguish one couple from another hailing from the Minnesota tundra. However, all participants came from the same Midwestern background, limiting the impact of environmental as well as income, cohort, or socioeconomic factors.

The explanation preferred by Zajonc and his colleagues is based on empathic responding and its effects on facial musculature. Most couples share some modicum of empathy for one another. The joys and sorrows of one partner typically are matched in the expression of the other partner, such that when one spouse beams with news of a pleasant event the other spouse responds in kind, grinning from ear to ear. This matching of facial actions—expressions of happiness at another's happiness, sadness at a partner's sorrow, and all points in between—multiplied by 25 years, may indeed etch the kinds of muscular and skin tone changes that produce greater resemblance. Bolstering this explanation, Zajonc found that when the original partners who supplied the photographs were surveyed, those who bore the greatest resemblance (and hence, who were likely to have experienced greater empathic responding) rated themselves as more satisfied with their relationships.

Besides being a novel take on the similarity thesis, Zajonc's study has important implications for your students. Ask them to conjure an image of the person they're currently dating or interested in. Concentrate on it......mull it over.....take a good long look: Do they really want to look like that person 25 years from now?!

Zajonc, R. B., Adelmann, P. K., Murphy, S. T., & Niedenthal, P. M. (1987). Convergence in the physical appearance of spouses. Motivation and Emotion, 11, 335-346.