People often think of love and lust as polar opposites—love exalted as the binder of two souls, lust the transient devil on our shoulders, disturbing and disruptive. Now neuroscientists are discovering that lust and love work together more closely than we think. Indeed, the strongest relationships have elements of both.

The bifurcated treatment of love and lust dates to antiquity. The study of love as an academic subject is nearly a century old, with the sentiment covered in introductory textbooks of social psychology. Psychologists, primatologists, neuroanatomists and neurophysiologists came to see love—defined as an intense and complex feeling of deep affection—as responsible for long-term coupling and close relationships. The first psychological tools for measuring love appeared in the 1940s. In a review of the literature published in 2011, psychologist Elaine Hatfield and her colleagues at the University of Hawaii at Manoa identified 33 scales for measuring love's gradations.

In contrast, researchers have traditionally regarded lust as little more than uncontrolled sexual urges. The scientific study of lust remained verboten or limited to clinicians, psychiatrists and sex therapists dealing with social and behavioral problems. When the topic of lust did appear in the scientific literature, it was cast as an archaic emotion, a sinful feeling that needed to be suppressed or denied lest it challenge societal order, or an addiction that hijacked human thought, emotion and behavior in insidious ways.

Now, though, neuroimaging investigations are beginning to flesh out the relationship between lust and love. Some research does support the Jekyll and Hyde dichotomy. Studies have revealed that lust and love both have unique brain signatures, suggesting they are separable, with the brain able to generate lust in the absence of love and vice versa. In one study of 500 individuals conducted in the mid-1960s by psychologist Dorothy Tennov of the University of Bridgeport, 53 percent of the women and 79 percent of the men agreed with the statement “I have been sexually attracted without feeling the slightest trace of love”; 61 percent of the women and 35 percent of the men agreed with the statement “I have been in love without feeling any need for sex.” Neuroimaging studies have also shown considerable overlap between the network for lust and the network underlying addiction, suggesting that the craving associated with lust brings with it impulsivity, lack of self-control and risk taking.

Other studies reveal a more complex and synergistic connection between lust and love. Both feelings can activate regions in the brain related to emotions, including euphoria, reward, motivation, addiction and body image. What is more, lust and love activate different parts of the same brain structures, the insula and the striatum.

A recent meta-analysis that we conducted of 20 studies with a total of 429 participants revealed that the posterior region of the insula activates more for lust than love, and the anterior region of the insula activates more for love than lust. This back-to-front distinction is in line with a broader principle of brain organization: posterior regions are involved in current, concrete sensations, feelings and responses, and anterior regions are involved in the integration of abstract concepts ranging from the distant past to alternative futures. In this model, lust would be grounded in particular sensory and motor experiences, with love as a more abstract, future-oriented gloss on those experiences with another person.

Studies show that as lust progresses to love, activity cascades from the back of the insula to the front, with the pleasing sensations of lust (sparked at the back) joined by the abstract feelings of affection (triggered at the front). A similar pattern for lust and love emerges in the striatum, this time traveling from bottom to top.

The research suggests that the strongest relationship—passionate love—involves activation of the home bases of both love and lust. Passionate love builds on the neural circuitry for lust, adding regions associated with reward expectancy, habit formation, and abstract representation and control to those associated with rewards for sensations and the satisfaction of cravings.

For any two individuals, the strongest relationship is not necessarily the best outcome: some couplings are just meant to be one-night stands. Love and lust can exist in any combination, with either, both or neither emotion present, and present to any degree. The combinations result in a variety of affiliations. When both people feel the same emotions, the relationship can range from passionate love (high love, high lust) to acquaintanceship (a little of each), with one-night stands (high lust, little love) and companionate love (as in a friendly marriage) in the middle. When the feelings of two people diverge, the results may be unwanted attention for one and unrequited love or lust for the other. The ideal state in any pairing is when the two people agree on their love-lust formula, creating a healthy balance between love and desire and the best chance for a stable, satisfying, monogamous relationship. But whatever the end point, getting there is half the fun.