Where Does Love Begin? The neurological origins of passionate love may begin in infancy, in the baby's attachment to the mother. In the view of evolutionary psychologists, maternal and romantic love, the deepest of human attachments, share a common evolutionary purpose—preserving the species—and so they share common neural mechanisms, the ones that make attachment and pair-bonding feel good. Key neurotransmitters and hormones that are involved in pleasure and reward are activated in the mother–baby pair-bond and again later in the pair-bond of adult lovers (Diamond, 2004).

Two important hormones for social bonding are vasopressin and oxytocin, which play a crucial role in the attachment–caregiving system, influencing feelings and expressions of love, caring, and trust not only between mothers and babies but also between friends and between lovers (Poulin, Holman, & Buffone, 2012; Walum et al., 2008). In one study, volunteers who inhaled oxytocin in a nasal spray were later more likely than control subjects to trust one another in various risky interactions (Kosfeld et al., 2005). In another study, couples given oxytocin increased their nonverbal expressions of love for one another—gazing, smiling, and fondling—in contrast to couples given a placebo (Gonzaga et al., 2006). Conversely, when prairie voles, a monogamous species, are given a drug that blocks oxytocin, they continue to mate, but they don't get attached to their partners (Ross et al., 2009).

These findings have inspired some oversimplifiers to call oxytocin the “love” or “cuddle” hormone, or even “liquid trust.” Cute, but if it really is such a spur to love and attachment, why are humans fighting so much? It turns out that giving people doses of oxytocin makes them more likely to favor their own group over other groups, and increases defensive aggression against outsiders (De Dreu et al., 2011). So perhaps oxytocin is the “cuddle your own kind and the hell with the rest of you” hormone. Moreover, high levels of oxytocin in women and of vasopressin in men are actually biological markers of relationship distress (Taylor, Saphire-Bernstein, & Seeman, 2010). Finally, people's love and attachment histories affect how they respond to oxytocin as well as the other way around (DeWall et al., 2014). After inhaling oxytocin, men who had had good attachments to their mothers remembered their moms as being unusually caring and supportive compared to men with good attachments who were given a placebo, whereas men who had troubled early home lives remembered their moms as being much less caring than similar men who received a placebo (Bartz et al., 2010).

The Role of Endorphins Some of the characteristic feelings and actions that occur during attachment are mediated by reward circuits in the brain and involve the release of endorphins, the brain's natural opiates. When baby mice and other animals are separated from their mothers, they cry out in distress, and the mother's touch (or lick) releases endorphins that soothe the infant. But when puppies, guinea pigs, and chicks are injected with low doses of either morphine or endorphins, the animals show much less distress than usual when separated from their mothers; the chemicals seem to be a biological replacement for mom (Panksepp et al., 1980). And when mice are genetically engineered to lack certain opioid receptors, they become less attached to their mothers and do not show signs of distress when separated from them. These findings suggest that endorphin-stimulated euphoria may be a child's initial motive for seeking affection and cuddling—that, in effect, a child attached to a parent is a child addicted to love. The addictive quality of adult passionate love, including the physical and emotional distress that new lovers feel when they are apart, may involve the same biochemistry (Diamond, 2004).

Using fMRI, neuroscientists have found other neurological similarities between infant–mother love and adult romantic love. Certain parts of the brain are activated when people look at images of their sweethearts, in contrast to other parts that are activated when they see pictures of friends or furniture. And these are the same areas that are activated when mothers see images of their own children as opposed to pictures of other children (Bartels & Zeki, 2004).

Clearly, then, the bonds of attachment are biologically based. Yet, as we saw with oxytocin, it is important to avoid oversimplifying, such as by concluding that “love is all in our hormones” or “love occurs in this corner of the brain but not that one.” Human love affairs involve many other factors that affect whom we choose, how we get along with that person, and whether we stay with a partner over the years (perhaps even 90 years!).

The Attachment Theory of Love After you find someone to love, how do you love? According to Phillip Shaver and Cindy Hazan (1993), adults, just like babies, can be secure, anxious, or avoidant in their attachments. Securely attached lovers are rarely jealous or worried about being abandoned. They are more compassionate and helpful than insecurely attached people and are quicker to understand and forgive their partners if the partner does something thoughtless or annoying (Konrath et al., 2014; Mikulincer & Shaver, 2007). Anxious lovers are always agitated about their relationships; they want to be close but worry that their partners will leave them. Other people often describe them as clingy, which may be why they are more likely than secure lovers to suffer from unrequited love. Avoidant people distrust and avoid intimate attachments.

Where do these differences come from? According to the attachment theory of love, people's attachment styles as adults derive in large part from how their parents cared for them (Simpson & Rholes, 2015). Children form internal “working models” of relationships: Can I trust others? Am I worthy of being loved? Will my parents leave me? If a child's parents are cold and rejecting and provide little or no emotional and physical comfort, the child learns to expect other relationships to be the same. In contrast, if children form secure attachments to trusted parents, they become more trusting of others, expecting to form other secure attachments with friends and lovers in adulthood (Feeney & Cassidy, 2003; Milan, Zona, & Snow, 2013). However, a child's own temperament and genetic predispositions could also help account for the consistency of attachment styles from childhood to adulthood, as well as for the working models of relationships that are formed during childhood (Fraley et al., 2011; Gillath et al., 2008). A child who is temperamentally fearful or difficult, or whose reward circuits do not function normally, may reject even the kindest parent's efforts to console. That child may therefore come to feel anxious or ambivalent in his or her adult relationships.

The Minnesota Longitudinal Study of Risk and Adaptation has followed a large sample of children from birth to adulthood, to see how early attachment styles can create cascading effects on adult relationships. People who are treated poorly early in life and lack secure attachments may end up on a pathway that makes committed relationships difficult. As children, they have trouble regulating negative emotions; as teenagers, they have trouble dealing with and recovering from conflict with their peers; as adults, they tend to “protect” themselves by becoming the less-committed partner in their relationships. If these individuals are lucky enough to get into a relationship with a securely attached partner, however, these vulnerabilities in maintaining a stable partnership can be overcome (Oriña et al., 2011; Simpson, Collins, & Salvatore, 2011; Simpson & Overall, 2014).

The Ingredients of Love When people are asked to define the key ingredients of love, most agree that love is a mix of passion, intimacy, and commitment (Hutcherson, Seppala, & Gross, 2015; Lemieux & Hale, 2000). Intimacy is based on deep knowledge of the other person, which accumulates gradually, but passion is based on emotion, which is generated by novelty and change. That is why passion is usually highest at the beginning of a relationship, when two people begin to disclose things about themselves to each other, and lowest when knowledge of the other person's beliefs and habits is at its maximum, when it seems that they have nothing left to learn about the beloved. Nonetheless, according to an analysis of a large number of adult couples and a meta-analysis of 25 studies of couples in long- and short-term relationships, romantic love can persist for many years and is strongly associated with a couple's happiness. What diminishes among these happy couples is that part of romantic love we might call obsessiveness, constant thinking and worrying about the loved one and the relationship (Acevedo & Aron, 2009).

Biological factors such as the brain's opiate system may contribute to early passion, as we noted, but most psychologists believe that the ability to sustain a long and intimate love relationship has more to do with a couple's attitudes, values, and balance of power than with genes or hormones. One of the most important psychological predictors of satisfaction in long-term relationships is the perception, by both partners, that the relationship is fair, rewarding, and balanced. Partners who feel overbenefited (getting more than they are giving) tend to feel guilty; those who feel underbenefited (not getting what they feel they deserve) tend to feel resentful and angry (Galinsky & Sonenstein, 2013; Pillemer, Hatfield, & Sprecher, 2008). A couple may tootle along comfortably until a stressful event—such as the arrival of children, serious illness, unemployment, or retirement—evokes simmering displeasure over issues of “what's fair.”

Another key psychological factor in couples' ability to sustain love is the nature of their primary motivation to maintain the relationship: Is it positive (to enjoy affection and intimacy) or negative (to avoid feeling insecure and lonely)? Couples motivated by the former goal tend to report more satisfaction with their partners (Gable & Poore, 2008). We will see that this difference in motivation—positive or negative—affects happiness and satisfaction in many different domains of life.

The critical-thinking guideline “define your terms” may never be more important than in matters of love. The way we define love deeply affects our satisfaction with relationships and whether our relationships last. After all, if you believe that the only real love is the kind defined by obsession, sexual passion, and hot emotion, then you may decide you are out of love when the initial phase of attraction fades, as it eventually must—and you will be repeatedly disappointed. Robert Solomon (1994) argued that “We conceive of [love] falsely. . . . We expect an explosion at the beginning powerful enough to fuel love through all of its ups and downs instead of viewing love as a process over which we have control, a process that tends to increase with time rather than wane.” However, people fall in love in different ways: Some couples do so gradually, over time, after “falling in friendship” first; and couples in arranged marriages may come to love each other long after the wedding (Aron et al., 2008). All the fMRIs in the world can't capture the rich variety of how people grow to love one another. For more discussion of these and related topics, watch the video Meeting Our Needs.