Here's a simple and sad fact: A lot of people who are married, or in long-term relationships, are not very compatible. Partners disagree about very basic stuff, like religion and politics and values, or they simply don't find each other attractive. Just look at the divorce statistics.

This raises a knotty and important question. If choosing a partner is such an important life decision, why do so many of us get it wrong? Why does the reality of a relationship fail so often to match our ideals? Obviously there are a lot of little differences that emerge over time, and people do change, but it seems like we should at least get the fundamental issues straight.

Psychological scientists are very interested in this question, but most have focused on self-focused errors in romantic choice. That is, we choose romantic partners who are rich or beautiful or fertile or otherwise valuable, but these qualities may not always make for a deep and enduring relationship.

This reasoning also assumes that we simply reject any potential partner who doesn't match our ideals. But do we? A team of researchers at the University of Toronto is offering a radical new idea about why we make so many poor relationship choices: We're too nice. According to Samantha Joel and her colleagues, the human mind has strong and automatic prosocial tendencies -- we don't like inflicting social pain -- and this deep-rooted kindness keeps men and women from rejecting partners -- even incompatible partners. What's more, we are unaware of our generosity's power. We think we will be picky about our romantic partners, but in reality, rejecting people is easier said than done.

At least that's the theory, which the Toronto scientists have been exploring in the lab. Here's how:

They recruited young men and women who were single but interested in dating, and showed each of them three dating profiles. These profiles were ostensibly of other people in the study. Each participant chose the potential date he or she preferred -- much as you would on a dating site. After the participants made their choices, they were given additional information about the person, including a photo that showed an unattractive man or woman. All were asked if they wanted to exchange contact information with this person. In other words, were they interested in the possibility of a date?

But here's the important part of the experiment: Some of the participants were told that their potential date was somewhere in the lab, available to meet now. Others were told to imagine that this potential date was nearby and available. The scientists were trying to distinguish here between how people see themselves choosing a partner, hypothetically, and how they actually choose in real time. They predicted that the young men and women would be much less picky -- less rejecting -- when they thought a real person's feelings were on the line.

And that's exactly what they found. Only one in six opted to date the unattractive person when it was a hypothetical decision. They saw themselves as choosy. By contrast, more than a third said yes to a date when they thought the unattractive person was in the next room. Importantly, the scientists asked the participants afterward about their motives for making the choices they did. Were you worried about the other person's feelings? Was guilt a factor? Or was this person a good match for you? Did you think he or she would be fun? They found that people were motivated by both self-interest and generosity of spirit. They were more excited about imminent dates (as opposed to hypothetical), but above and beyond that, they were more concerned about the other's feelings than they thought they would be.

So we've all heard uplifting stories about the most popular student on campus going to the big dance with the ugly duckling or the awkward nerd. But that noble gesture is usually a one-time act of pity, and besides, we know the unattractive loser is really brilliant and funny and so forth. But what if physical attractiveness is not a factor? What if the incompatibility runs deeper than that, to core values?

Joel and her colleagues ran a second experiment to explore this question. It was for the most part identical to the study just described, except that instead of receiving an unattractive photograph, the participants learned about undesirable habits or traits of the potential date. These traits were previously identified by the participants as "deal killers" -- differences on matters of politics or religion or values that were grounds for rejection. As before, some made a hypothetical choice based on this information, while others made what they thought was a real-life dating decision.

And the results were the same. As reported in a forthcoming article in the journal Psychological Science, participants in the real-life situation were significantly less likely to reject a potential date based on deal-killing character traits. Indeed, three-quarters of the participants who knew of these deal-killing traits opted to date the person anyway. And again, they were motivated by a reluctance to hurt another person's feelings.

So is this a good thing? Well, it's encouraging that people are so powerfully motivated by empathy and kindness, but what if these emotions are contributing to unhappy relationships? It's not clear from these studies just how far people would be willing to go to accommodate undesirable suitors. It's plausible, the scientists say, that sparing others' feelings may become less important as the costs increase -- the costs of long-term commitment, for example. On the other hand, empathy can grow as people become closer and more intimate. So it's possible that, the more we invest in a relationship, the less we want to hurt our partners -- and the more likely we are to stay.

Follow Wray Herbert's reporting on psychological science in The Huffington Post and on Twitter at @wrayherbert. Follow Samantha Joel's work at www.samjoel.com and on Twitter at @datingdecisions.