By RICHARD A. FRIEDMAN

Published: November 2, 2012

TO love is to suffer; to be happy is to love. So must one suffer to be happy? This syllogism won’t win any prizes in logic, but it accurately describes a curious paradox of human behavior: the allure of unpredictable romantic partners.

We’ve all been told that faithfulness and constancy are desirable and even virtuous, yet we have been warned by our poets and philosophers that it’s an uphill battle against the fickleness of love. It’s been 400 years since Shakespeare warned women that “men were deceivers ever; one foot in sea and one on shore, to one thing constant never.”

We seem to enjoy Shakespeare’s words more than we heed them, considering how commonly people complain that they love someone who always disappoints them.

For psychoanalysts, people who seem drawn to adversity, whether in relationships or in other areas of life, raise the legitimate question of whether they harbor an unconscious motive for suffering.

But I think there might be another way to understand the attraction of unpredictable romantic partners, one that involves a quirk of the brain’s reward circuit, a primitive neural network buried deep in our brains that is exquisitely sensitive to various rewards, like sex, money and food.

This kind of amorous attachment is like gambling — except that the currency is affection and sex. The key is that the reward is unanticipated, which makes it particularly powerful and alluring to our brains.

To understand why, consider what happens in the brain when people are given rewards under two different conditions: predicted and unpredicted. The psychiatrist Gregory Berns did just that in a study in which subjects were given fruit juice and water, both naturally pleasurable rewards, while scanning their brains with an M.R.I. During part of each session, subjects received water and fruit juice at random intervals; during another part, the water and juice were administered every 10 seconds.

Professor Berns discovered that the water and juice elicited greater activation in the brain’s reward circuit when the reward was unanticipated than when it was delivered in a predictable fashion. The pattern held true whether the reward was water or fruit juice — even though most subjects claimed a clear preference.

When the reward circuit fires, it also tells the brain something like, “Pay attention and remember this experience because it’s important.” This circuit releases dopamine when stimulated, which, if it reaches a critical level, conveys a sense of pleasure.

The reason this happens is simple. The brain’s reward circuit has evolved over millions of years to enable us to recognize and extract various rewards from our environment that are critical to our survival, like food and a suitable sexual mate. Unlike predictable stimuli, unanticipated stimuli can tell us things about the world that we don’t yet know. And because they serve as a signal that a big reward might be close by, it is advantageous that novel stimuli command our attention.

Which brings us to inconstant love. It turns out that human love and attachment are, like the fruit juice in Professor Berns’s experiment, natural reinforcers that can activate your reward pathway. The anthropologist Helen Fisher studied a group of 17 people in the grip of intense romantic love and found that an image of their beloved strongly activated the reward circuit.

If you are involved with someone who is unpredictably loving, you might not like it very much — but your reward circuit is sure going to notice the capricious behavior and give you information that might conflict with what you believe consciously is in your best interest.

Indeed, you may not even be aware of your own reward circuit’s activity. One of the curious things that Professor Berns found was that most of his subjects couldn’t tell the difference between the predictable or unpredictable condition in which the reward was given.

Since unpredictable rewards cause more dopamine release than predictable ones and more dopamine means more pleasure, one implication of this study is that people experience more pleasure with unpredictable rewards than with predictable ones — but they may not be consciously aware of this fact.

Not just that, but there was essentially no relationship between the subjects’ stated preferences and the observed activity in their reward circuit. This suggests that our reward pathways may not only be activated without our recognition, but perhaps even in ways that are contrary to what we think we prefer.

These data might explain, in part, the paradox of people who complain constantly about their unreliable lovers, but keep coming back to them, time and again.

It might also explain some famously bad behavior, like King Lear’s ill-treatment of Cordelia. Unfortunately for Cordelia, her father knew he could count on the love of his devoted and constantly affectionate daughter. Compared with her scheming sisters, Cordelia was just not all that exciting — at least to Lear’s reward circuit.

None of this is to say that just because our reward circuits light up in the face of unanticipated rewards, that we are off the hook. Far from it. We use conscious knowledge to override our unhealthy or undesirable impulses all the time. Except for a few limited circumstances, we are expected to be in charge of our brains.

Still, it should help us understand those friends who find themselves drawn to unpredictable romantic partners. They are not necessarily gluttons for pain or disappointment; they might be addicted to the hidden pleasure of inconstant love.

Richard A. Friedman is a professor of clinical psychiatry and the director of the Psychopharmacology Clinic at the Weill Cornell Medical College.