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Love is universal, or at least it seems to be.

Every society has a romantic tradition, and the desire to form a long and (hopefully) lasting love with another human being is apparently an intrinsic part of being human. But while seems to be the ideal for most people, attaining that ideal is easier said than done. Like it or not, humans aren't likely to stay with one romantic partner for their entire lives. No matter how a society chooses to define marriage, nearly half of all such partnerships end in .

This is hardly unique to modern society: According to David Buss, hunter-gatherer societies seem particularly prone to divorce. For example, members of the Ache society of Paraguay can report as many as 11 marriages by the time they reach adulthood.

For whatever reason, most are going to come to an end, whether harmoniously or not.

Though we recognize that love is often fragile, it can be difficult to understand exactly why a romantic relationship comes to an end. For most people, this can be a experience, especially for the one who is caught unaware that a relationship is over. The shock and that can result from a bad breakup can take years to work through; learning to move on can be extremely difficult.

While there has been extensive research into the psychology of romantic love, is it possible to learn what can cause people to fall out of love with a significant other? For that matter, how is it possible to move on after such a relationship comes to an end? A new article published in Review of General Psychology suggests that humans may actually have a mental mechanism in place for severing the emotional bond between romantic partners. Authors Brian Boutwell of Saint Louis University, J.C. Barnes of the University of Cincinnati, and Kevin Beaver of Florida State University suggest in their article that their concept of a "mate rejection module" is hardly new.

Evolutionary psychologists have long argued that the ability to end a relationship and prepare for a new one can have definite advantages in terms of improving the ability to reproduce successfully. While some species can mate for life, humans typically don't. Admittedly, there are enormous differences across cultures—and at different periods in history—which makes it difficult to pin down how the mate rejection module actually works. Not only have some cultures allowed multiple marriages, but ending a marriage has been extremely difficult in many times and places. Even with casual relationships that don't involve any formal declarations, moving on from one partner to the next one can still be tricky.

In their review, Boutwell and his co-authors suggest that there are can be separate mechanisms at work:

First is what they called primary mate ejection, or the active decision to reject a mate.

Secondary mate ejection involves coming to terms with being rejected and reaching the point at which a new romantic pairing becomes possible.

Not surprisingly, there appear to be significant differences between males and females in terms of where and when their mate ejection modules are activated. When dealing with , for example, men and women may react very differently, depending in part on how serious the infidelity is considered to be. Since, in evolutionary terms, men are more sensitive to the possibility of raising someone else's child, they may be less prone to forgive sexual infidelity than women. On the other hand, women seem less prone to emotional infidelity—such as when a partner falls in love with another woman, rendering him no longer a "safe bet" in terms of supporting her children (or her). Studies examining how willing men and women are to forgive infidelity appear to support this difference.

The kind of resources that a woman has, and the opportunities within the culture in which she lives, can also affect how willing she is to eject a mate. If she is able to support herself, the decision to leave is often easier than if she might be left destitute following a split.

But is it possible to use to understand how mate ejection works for men and women?

In their article, Boutwell and his colleagues examined some of the neurological underpinnings of romantic love, including fMRI studies showing that many of the brain pathways linked to being in love show strong similarities with pathways linked to . (Didn't you always suspect that?)

These pathways include the orbitofrontal and prefrontal cortex, right ventral tegmental area, and the ventral striatum, all of which involve reward or pleasure areas of the brain. The love-addiction link also helps explain the acute distress people feel when rejected by a romantic partner, as well as why stalkers have so much trouble letting go of their romantic feelings.

While the feeling associated with mate ejection can resemble drug withdrawal in many ways, both primary and secondary mate ejection take place after a gradual process of overcoming these old feelings and preparing the brain for to a new partner. Once mate ejection occurs (whether primary or secondary), parts of the prefrontal cortex begin the process of "learning" to prepare for a new romantic interest.

(An intriguing example of how brain biochemistry can affect mate ejection involves research looking at (SSRI) anti-depressants. By suppressing serotonin levels, the brain's and norepinephrine levels are affected as well. Not only does this lower levels, but it may also suppress romantic feelings. For men, this might trigger a preference for sexual variety and, as a result, increase the risk of infidelity. For now, Boutwell et al. simply raise this as a possible explanation for why men may become more sexually promiscuous as a side effect of using these medications, but it can also be help understand how biochemical changes could lead to relationship problems.)

There remain numerous unanswered questions about mate ejection:

Is it the same for both and heterosexual relationships? While early studies suggest that it is, this is still largely open to debate.

Then there is the question of how mate ejection can be measured. Can the same brain pathways that seem to be involved in romantic love be linked to the decision to end a relationship?

And what about cases of obsessive love, such as erotomania? Could understanding more about mate ejection help with treatment of stalkers?

All of these questions are potential areas for future research.

"Til death do us part" pairings do exist, of course, but for whatever reason, most romantic relationships will eventually come to an end. Whether the breakup that ends a love affair is mutual or one-sided, we all have the capacity to move on to someone else, even if coping with heartache and disappointment can be overwhelming at times.

In many ways, ending a love affair is a lot like overcoming a drug addiction—and brain research seems to bear this out. However we first evolved the capacity for mate ejection, it seems to have become an important part of the human condition. Learning more about mate ejection may also provide the key to understanding how to keep love alive for as long as humanly possible.