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.