Likewise, dating sites don’t take into account the environment surrounding the relationship: factors like job loss, financial strain, infertility and illness. But research indicates that when couples encounter such stresses or unexpected demands on their energy, their satisfaction with their relationship declines and their risk for breaking up increases. To give just one example: in a 2004 study by the psychologist Lisa Neff, wives who experienced relatively high levels of stress outside of their marriage tended to evaluate their marriage increasingly negatively over time.

Another major problem with the algorithms of dating sites is that the information that they do collect — about individual characteristics — accounts for only a tiny slice of what makes two people suited for a long-term relationship. Certainly, some characteristics predict relationship well-being. For example, decades of research confirms that people tend to have troubled romantic relationships if they are emotionally volatile, were mistreated as children or abuse drugs or alcohol. Eliminating people from the dating pool who are likely to have relationship problems, as some sites may do by declining customers based on their answers to questions about things like emotional stability, can be a useful service (as long as you’re one of the lucky singles who make the cut).

Of course, dating sites promise much more than access to a somewhat improved pool of potential mates; they promise to identify specific pairs of strangers who are likely to mesh well together in a romantic relationship. In particular, almost all of the sites claim that partners who are more similar to each other in certain ways will experience greater relationship satisfaction and stability relative to partners who are less similar.

But our review of the literature revealed that the forms of similarity advertised by dating sites provide a meager foundation for an enduring relationship. To be sure, similarity on some dimensions, like race and religion, does predict relationship well-being. Analyses by the National Center for Health Statistics, for example, indicate that marriages between spouses of the same race or ethnicity have a lower divorce rate after 10 years than interracial or interethnic couples (31 percent versus 41 percent). However, the vast majority of people mate with demographically similar partners anyway, so such findings aren’t especially useful in helping dating sites narrow a client’s pool of potential partners.

Perhaps as a result, these sites tend to emphasize similarity on psychological variables like personality (e.g., matching extroverts with extroverts and introverts with introverts) and attitudes (e.g., matching people who prefer Judd Apatow’s movies to Woody Allen’s with people who feel the same way). The problem with this approach is that such forms of similarity between two partners generally don’t predict the success of their relationship. According to a 2008 meta-analysis of 313 studies, similarity on personality traits and attitudes had no effect on relationship well-being in established relationships. In addition, a 2010 study of more than 23,000 married couples showed that similarity on the major dimensions of personality (e.g., neuroticism, impulsivity, extroversion) accounted for a mere 0.5 percent of how satisfied spouses were with their marriages — leaving the other 99.5 percent to other factors.