Online dating is one of the fastest growing industries, and many websites now include some sort of profiling tool for predicting romantic compatibility. However, few of these websites (and that is also true for most researchers working in this area) have taken the time to consider the intricate issue of predicting compatibility, namely that there is no simple way of defining relationship success.
Indeed, how you judge the success of a relationship depends on several psychological and cultural factors, not to mention age and gender. In the Western world, most people—at least those aged 25-40 (who represent the main target of most online dating sites)—equate success to a happy relationship, a memorable wedding, and healthy children. Yet marriages are at an all-time-low, and if divorce rates continue to increase the way the have for the past 20 years, then only a minority of couples can expect to be together for over 15 years... let alone "forever".
Furthermore, what people prioritise (among the common criteria) depends on who they are, what their values are, and where they live. For example, some people may prefer a longlasting relationship with no children to a shortlasting one with children; others, a short passionate relationship to a long, dispassionate one; and some may prefer a relationship that is compatible with their career plans (and financially advantageous) to one that is not, even if the latter is based on love.
The question, then, is not how to predict compatibility, but how best to define it—and the first step to answer this question is to look at whether the two people in a relationship share compatible views on what a successful relationship should be. Clearly, if they disagree on what they need, they will only be happy by accident.
In the event that two potential daters are "in sync" about what they expect to get from the relationship - assuming they share the same goals (e.g., longevity, children, financial success, passion, adventure, etc.)—there are five basic ways of being incompatible, and personality plays a role in all of them.
First, they may differ in ambition: If one of the two people is insanely driven and the other one totally laid-back, the relationship wont work. One would regard the other one as "type A" and too intense, and the other would regard him or her as too lazy and a even a freeloader. Thus career focus and achievement motivation should be failry balanced among daters (even if they have different careers, they should be equally ambtious)
Second, daters may differ in openness: if one of them has a "hungry mind" but the other one has no interests in culture or knowledge, they will struggle. Indeed, highly open people are constantly trying out new things: they love travelling to new places, trying out new foods, and doing unusual things. Conversely, people with low openness are conservative, risk-averse, and will end up seeming boring to their more curious partners.
Third, daters may differ in emotional intelligence: if this difference is minor, it will be tolerated. However big differences will cause one of the partners (the less emotionally stable one) to use the other one as a shrink! Indeed, stable people are the perfect target for neurotic partners, because they have sedative effects on them! So, if you are highly emotionally stable and single, beware of needy, neurotic, people!
Fourth, daters may differ in sociability: again, reasonable differences may be tolerated—and most couples, even successful ones, have partners with different levels of extraversion. However if one of the two is much more sociable than the other, s/he will be much more interested in meeting new people, going out to bars and parties, and spending a great deal of time advertising his/her private life on Facebook - in contrast, the other person will want a private, quiet and personal relationship (which will bore the extraverted partner to death!)
Last, but not, least, partners may differ in altruism or agreeableness: indeed, if one of them has high interpersonal sensitivity but the other one has low empathy levels, they will end up behaving in very different ways—and, especially the more sensitive partner, will find the selfish behaviour of his/her partner borderline rather immoral: whether its recycling or cycling, giving to charity or donating blood, partners should be equally predisposed to helping others... or they will experience cognitive dissonance.
The above list of traits is by no means comprehensive; and even when combined personality may only explain a relatively small percentage of variance in romantic compatibility. Yet, just like businesses pay attention to personality when selecting employees, you should pay attention to personality (yours and that of your potential partner) before investing on romantic relationships.
If you want to find out more about what type of person you are in relationships take this quiz.