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The Evolution of Love

June 12, 2011, 12:00 AM
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How did we evolve the most loving brain on the planet?

Humans are the most sociable species on earth – for better and for worse.

On the one hand, we have the greatest capacities for empathy, communication, friendship, romance, complex social structures, and altruism. On the other, we have the greatest capacities for shaming, emotional cruelty, sadism, envy, jealousy, discrimination and other forms of dehumanization, and wholesale slaughter of our fellow humans.

In other words, to paraphrase a Native American teaching, a wolf of love and a wolf of hate live in the heart of every person.

Many factors shape each of these two wolves, including biological evolution, culture, economics, and personal history. Here, I’d like to comment on key elements of the neural substrate of bonding and love.

These are complex subjects, so I hope you’ll forgive some simplifications. Here we go.

Evolution

The growing length of childhood coevolved with the enlarging of the brain – which has tripled in size over the last 2.5 million years, since the time of the first tool-making hominids – and with the development of complex bonding, which includes friendship, romantic love, parent-child attachment, and loyalty to a group.

As the brain grew bigger, childhood needed to be longer since there was so much to learn. To keep a vulnerable child alive for many years, we evolved strong bonds between parents and children, between mates, within extended family groups, and within bands as a whole – all in order to sustain “the village it takes to raise a child.” Bands with better teamwork outcompeted other bands for scarce resources; since breeding occurred primarily within bands, genes for bonding, cooperation, and altruism proliferated within the human genome.

Numerous physical, social, and psychological factors promote bonding. Let’s focus on physical factors, and then drill down further to examine two chemicals inside your brain: dopamine and oxytocin. Both are neurotransmitters, and oxytocin also functions as a hormone when it acts outside the nervous system.

(By the way, dopamine and oxytocin, like many other biochemical factors, are present in other mammals, too, but as with most things human, their effects are much more nuanced and elaborated with us.)

Dopamine

It’s an error to reduce love to chemicals, since so many other factors are at work in the brain and mind as well, so let’s hold this material in perspective.

That said, it appears that when people are in love, among other neurological activities, two parts of their brain really get activated. They are called the caudate nucleus and the tegmentum. The caudate is a reward center of the brain, and the tegmentum is a region of the brain stem that sends dopamine to it; dopamine tracks how rewarding something is.

In effect, being in love rewards the pleasure centers in your brain, which then crave whatever it was that was so rewarding – in other words, your beloved. Those reward centers are the same ones that light up when people win the lottery. Or use cocaine.

And being rejected in love activates a part of the brain called the insula, which is the same region that lights up when we are in physical pain.

So we are doubly motivated to hold fast to the object of our love: feel the pleasure, and avoid the pain.

Interestingly, when people are in lust, rather than in love, different systems of the brain get activated, notably the hypothalamus and the amygdala.

The hypothalamus regulates drives like hunger and thirst. Interestingly, the word in the early records of the teachings of the Buddha that is translated in English as the “desire” or “attachment” or “clinging” that is the root of suffering has the fundamental meaning of “thirst,” so it’s pretty likely that the hypothalamus is involved in much of the clinging that leads to suffering.

The amygdala handles emotional reactivity, and both it and the hypothalamus are involved in arousal of the organism and readiness for action. (While these systems are centrally involved in fight-or-flight responses to stress, they also get engaged in energizing activities that feel emotionally positive like cheering on your favorite team – or fantasizing about your sweetheart.)

These neural components may shed some light on the subjective experience of being in love, which commonly feels softer, more “Aaaaahh, how sweet!” rather than the “Rawwrh, gotta have it!” intensity of lust.

That said, dopamine – increased in love – triggers testosterone production, which is a major factor in the sex drive of both men and women.

So, in short, we fall in love, and among other neural circuits and psychological complexities, the same reward chemicals involved in drug addiction lead us to crave our beloved and want sex with him or her. Sorry to be mechanistic here, but you get the idea.

The intended result, in the evolutionary playbook, is, of course, babies.

Then what?!

Oxytocin

Oxytocin promotes bonding between mothers and children, and between mates, so they work together to keep those kids alive.

For example, in women, oxytocin triggers the let-down reflex in nursing, and is involved in that blissful, oceanic feeling of peace and comfort and love experienced by many women while breastfeeding.

It also seems to be part of the female response to stress (more than in men – since women have much more oxytocin than men do), in part by encouraging what Shelley Taylor at UCLA has termed “tend-and-befriend” behaviors in women when they are stressed.

(Of course, men, too, will often reach out to others and be friendly during tough times, whether it’s crunch quarter at the office, or somewhere in a dusty war – another example of how there are many pathways in the brain to important functional results.)

The experiential qualities of oxytocin are pleasurable feelings of relaxation and rightness, so it is an internal reward for all bonding behaviors – not just with mates.

Oxytocin encourages sociability; for example, when oxytocin capabilities are knocked out in laboratory mice, their relationships with other mice are very disturbed.

And oxytocin dampens the stress response of the sympathetic nervous system and the hypothalamus-pituitary-adrenal axis – besides having functional benefits, this is another pathway for rewarding, and thus encouraging, bonding behaviors.

What triggers this warm and fuzzy and let’s-get-together-now chemical?

Oxytocin is released in both women and men:

• When nipples are stimulated (such as through nursing)

• During orgasm, promoting the afterglow of warm affection (and a tendency, sometimes annoying in a partner, to fall asleep!)

• During extended, physical, especially “skin-to-skin” contact (e.g., cuddling children, long hugs with friends, teens forming packs on the couch, lovers caressing after sex)

• When moving together harmoniously, like dancing

• When there are warm feelings of rapport or love; a strong sense of compassion and kindness probably entails releases of oxytocin, though I haven’t seen a study on that specific subject (a great Ph.D. dissertation for someone).

• Probably during devotional experiences, such as in prayer, or while with certain kinds of spiritual teachers

Probably, oxytocin can also be released just by imagining – the more vividly, the better – the activities just mentioned, particularly when combined with warm feelings.

* * *

Of course, dopamine and oxytocin are just two of the many factors at work in our relationships. For example, philosophical values or ideals of universal compassion, such as in the major religions of the world, can also influence a person’s behavior greatly, with or without any measurable surges of dopamine or oxytocin.

Nonetheless, appreciating the biochemical factors at work on Valentine’s Day, or at any time we experience bonding or love, can help a person not get quite so swept away by the ups and downs of relationships.

 

The Evolution of Love

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