When searching for a mate, most of us carry around a list of criteria or qualities that we tell ourselves we’re looking for. A brief perusal of any Internet dating site yields many such lists: “A sense of humor, a sense of adventure, and good teeth” says one. “Owns his own business, likes to travel, and works out,” says another. Curiously, however, we often end up with mates who seem to lack many if not most of the qualities we said we wanted.  Why is that?

One reason may be that people don’t come custom-order and we must all inevitably make compromises if we don’t want to end up alone. On the other hand, many “mate” choices people make are more than just a little surprising given the criteria they claimed to have been using. In some cases, it’s as if their shopping list was completely tossed aside.

In fact, this is often exactly what happens. As with everything else, our conscious minds play second fiddle to our unconscious desires. That is to say, we may think we know what we want in a mate, but the real qualities we find attractive—the real reasons for the choice we ultimately make—are often quite different from what we tell ourselves they are.

Or, at least, the relative importance of each quality we want is different.  To play into stereotypes for just a minute, maybe a woman’s physical beauty is so great we find ourselves completely ignoring that she never finished high school even though we thought we wanted a professional. Or a man’s wealth is so enormous we find ourselves ignoring that he drinks too much.

The problem, of course, is that our unconscious minds are frequently as poor at choosing a mate as are our conscious ones. Should we really value an adventurous spirit over honesty? Or good hygiene over fashion sense? Or common values over common interests? It really is a complex calculation we need to make when choosing a mate, made all the more difficult not only because no perfect answer ever exists, but also because no good, or even great, answer we come up with today is guaranteed to be a good, or even great, answer tomorrow. People often change over time in unpredictable ways,

But I would argue that there’s one variable almost none of us includes in these calculations that make all other variables almost insignificant: What kind of person does the person we choose turn us into?

The self-fulfilling prophecy theory of social interaction argues that the way we expect other people to behave alters our behavior in such a way that causes them to fulfill our expectations. This was demonstrated in a study by researcher Mark Snyder and colleagues when they randomly assigned fifty-one male undergraduates to look at one of eight photographs of female undergraduates (four of which had been rated as attractive and four of which had been rated as unattractive by other men previously) and asked them for their initial impressions. Once they’d confirmed the findings of previous research that showed men expected attractive women to be warmer than unattractive women, they asked the men to engage in telephone conversations (so they couldn’t see to whom they were talking) with the women whose photographs they’d seen.

Unbeknownst to the men, however, none of the women with whom they talked were the women in the photographs. When blinded observers then evaluated tape recordings of the conversations, they found that the men who spoke with women they thought were attractive (and who they therefore expected to be warm) were warmer in conversation than the men who spoke with women they thought were unattractive (who they therefore expected not to be warm), confirming that the expectations the men had of the women affected their own behavior. Even more interesting, though, was that the women who the men thought were attractive were also rated by the blinded observers as warmer in conversation than the women who the men thought were unattractive. Thus, the expectations that the men had for the women drove the way the men behaved toward the women, which in turn drove the way the women behaved toward the men.

Further, other research suggests that when we make our behavior conform to another person’s expectations, we tend to internalize those expectations, which makes us more likely to repeat that behavior in the future. Not only that, but we also tend to attribute our subsequent behavior not to previous expectations others have had of us but to our own disposition, especially if multiple people confirm our self-perception in multiple contexts. Thus, if our parents, our teachers, and our friends all treat us as if we’re helpless, helpless is what we’ll believe ourselves to be and thus what we’ll likely become.

How does all this apply to mate selection? Brad Pitt is reputed to have said about his divorce from Jennifer Aniston: “I didn’t like who I was when I was with her.” We spend most of our time around other people. If other people, by virtue of who they are, pull out of us people we ourselves don’t like, we’ll end up spending most of our time as people we don’t want to be. And arguably no one has a greater ability to impact who we are than our mate.

The problem, of course, is that who our mates pull out of us also changes over time. Initially, they likely pull out someone we like very much: our excited, passionate, happy selves. Later, however, as our relationship evolves, they may begin to pull out of us parts our ourselves we don’t like at all: our angry, demanding, or even depressed selves. Ultimately, of course, we bear the responsibility for who we are. But the way we influence who we are isn’t by simply deciding to be different. We have to be clever. We have to pull levers—arrange positive influences—that actually yield the changes we want. Who we choose to spend our lives with may be one of the most powerful influences of all. Though we can’t necessarily predict who they’ll pull out of us in the future, we can at least ask ourselves who they pull out of us now. And if we don’t like that person, no matter how much we may like the person we want to choose, perhaps we should think twice about making them our choice.

Alex Lickerman is an internal medicine physician at the University of Chicago who blogs at Happiness in this World.  He is the author of The Undefeated Mind: On the Science of Constructing an Indestructible Self.