The ongoing debate of whether opposites attract or like attracts like in relationships has both passed and failed the test of time. Typically, when we're looking to date, we're interested in partners who are fundamentally similar to ourselves — consistent in our desires, thoughts, and attitudes. But, once we're in a romantic relationship, how does it affect who we find attractive?

A study published in the journal Frontiers in Psychology found in relationships, we're more likely to be attracted to faces resembling our own, but for single people, opposites attract.

"We found that single participants, those not in relationships, rate dissimilar faces as more attractive and sexy than self-resembling faces," said Dr. Jitka Lindová, study author of Charles University in the Czech Republic, in a statement.

Lindova and her colleagues recruited university students to evaluate a series of photographs of faces and asked them to rate their attractiveness. They were also presented with images of a same-sex individual manipulated in the same way. Prior to looking at the photos, the participants were asked whether or not they currently had a romantic partner.

The two instructions for attractiveness ratings of the opposite sex were as follows: “Choose the man/woman (according to the sex of rater) which you find more attractive for a short-term/long-term romantic relationship.” The instructions for same-sex attractiveness ratings also had two variants: “Choose the man/woman (according to the sex of rater) which you think will be more attractive to the opposite sex” and: “Choose the man/woman (according to sex of rater) which you would prefer as your friend.”

The findings revealed single participants, not those in relationships, rated dissimilar faces as more attractive and sexy than those that resembled themselves. This effect was observed when participants rated both same-sex and opposite-sex faces.

"For the first time, we have observed how our partnership status affects who we find attractive;" said Lindova.

Previous research has found we prefer faces of unfamiliar individuals who are genetically similar to us. Experiments using morphing techniques to manipulate facial shape show we prefer similarity, not dissimilarity, in opposite-sex faces. However, little was known about how our relationship status affects the type of people we find attractive.

The researchers believe our preference for people similar to us is part of a relationship- maintenance strategy to prevent us from finding alternatives to our own partner. Couples seem to pay less attention to cues of sexual attractiveness in others compared to singles. Factors like potential kinship or friendship may have a heavier influence in facial judgements for committed couples.

Moreover, the preference for our partner to look like us could be associated with several benefits, including increased kinship support and positive social behavior.

Meanwhile, singletons’ desire for differences could be attributed to the idea they’re more actively searching for a mate, and they’re looking for someone with biological diversity who will be genetically suitable for producing offspring. This is because partners who look less like us are actually more genetically suitable for reproduction.

A similar study also finds our relationship status can make us view people whom we might otherwise find attractive as less good-looking. We tend to immediately find flaws in other objectively attractive people as a means to help us avoid temptation and stay satisfied in relationships. However, for people who are unhappy in their relationships, they’re more likely to find outside people more attractive, similar to how single people perceive them.

So, although monogamy isn't necessarily hard-wired within us, our brains do their best to help us stay committed in relationships.

Source: Lindova J, Little AC, Havlicek J. Effect of Partnership Status on Preferences for Facial Self-Resemblance. Frontiers in Psychology. 2016.