Jeremy Dean writes about a study of couples that suggested facial similarity between partners increases over time. Both the study's authors and Dean believe that "empathy" may account for this.
People grow to look similar because they are empathising with each other and so copying each other's facial expressions. Over time because of all the empathising they are doing, their faces come to look more similar."
I suspect that the processes behind converging appearance are far more complex than this explanation would suggest. The more broadly encompassing concept of intersubjectivity addresses the interpenetration of mental worlds extending well beyond empathy and constituent activities such as imitation.
Interpersonal relations entail an interplay of ongoing adjustments, assimilations, accommodations, micro-attunements and mis-attunements between partners. Presaging this understanding, Heinrich Racker wrote about the two-way nature of interpersonal traffic using the transferential terms concordant and complementary identifications.
Later, in The Interpersonal World of the Infant, Daniel Stern explored the mutuality of maternal-infant influences and regulatory activities within the mother-infant/toddler dyad. In a sense, this interplay of influences constitutes a more fluid, boundariless, shared mind that is continuous with the internal worlds of both mother and infant. Stern would say that it is within the context of this flowing, shared mental experience that the psychic world of the infant develops. Based upon his extensive, videotaped observations of mother-infant interactions, Stern offered a detailed description of the emergence of the self (really, layers of context-bound selves) within this relational matrix.
During infancy and toddlerhood, when neuroplasticity is greatest, the infant is highly responsive to relational cues and pressures that forge a sense of familiarity and continuity of inner experience, as well as patterns of outward expression. But the infant doesn’t just imitate the mother/caretaker as the process evolves. The infant experiences his or her own impulses, desires, frustrations and fears, engaging with the mother in ways that influence and shape the mother's experience, as well. Together, each partner to the dyad contributes to the formation of a sort of third, more transient dyadic-mind within which both minds engage in fluid, ongoing constructive and reconstructive processes. I don't have the reference handy, but I believe I've read that there is evidence that, after childbirth, women experience a temporary increase in neuroplasticity. If that’s true, it might enhance the mother’s flexibility in adapting to the influences and constitutional realities of the infant.
When we discuss convergence of appearance in an adult couple, the observations above are relevant since the intersubjective qualities of psychic life continue into adulthood. And, although neuroplasticity is reduced, it is far from absent. I've long suspected that couples undergo similar processes that could lead partners, over time, to look more alike. I also suspect that certain aspects of the appearance (and inner experience) of one partner might instead represent either complementary or projected unwanted psychic parts of the other partner.