One psychologist, Robert Zajonc of the University of Michigan conducted his popular ‘Facial Likeness’ study in 1987 and found that couples who have been together for as long as 25 years looked so similar in appearance that participants in the study did not have any trouble pairing them together despite having no prior knowledge of their marriage.
During the study, Zajonc and his team asked volunteers to look carefully at photographs of men and women and match them based on facial similarity. The researchers found out that couples who had been married for 25 years were rightly paired together.
An explanation for this, according to Zajonc, is that couples who have been together for that long would have had a lot of shared experiences that left similar lines on faces, and that couples would therefore begin to look more similar. In essence, couples would have had a lot of moments of shared laughter, sadness, and would have gone through a lot of emotions together, which leaves them with the same facial lines.
It has been discovered that as couples age together, they tend adapt themself and change in accordance with their partner. Hence, their life experiences are reflected in their faces. Couples that have shared a happy and joyful life together develop muscles and wrinkles related to smiling and laughter, whereas couples that have been unhappy have frowns, anger and depression reflected in their faces.
In another study, when partners look alike, it could have nothing to do with how long they have been together. According to a 2010 report from Personality and Social Psychology Bulletin, people tend to be attracted to members of the opposite sex who look like them or their parents.