Every great love affair begins with a scream. At birth, the brain starts blazing new neural pathways based on its odyssey in an alien world. An infant is steeped in bright, buzzing, bristling sensations, raw emotions and the curious feelings they unleash, weird objects, a flux of faces, shadowy images and dreams — but most of all a powerfully magnetic primary caregiver whose wizardry astounds.

Brain scans show synchrony between the brains of mother and child; but what they can’t show is the internal bond that belongs to neither alone, a fusion in which the self feels so permeable it doesn’t matter whose body is whose. Wordlessly, relying on the heart’s semaphores, the mother says all an infant needs to hear, communicating through eyes, face and voice. Thanks to advances in neuroimaging, we now have evidence that a baby’s first attachments imprint its brain. The patterns of a lifetime’s behaviors, thoughts, self-regard and choice of sweethearts all begin in this crucible.

We used to think this was the end of the story: first heredity, then the brain’s engraving mental maps in childhood, after which you’re pretty much stuck with the final blueprint.

But as a wealth of imaging studies highlight, the neural alchemy continues throughout life as we mature and forge friendships, dabble in affairs, succumb to romantic love, choose a soul mate. The body remembers how that oneness with Mother felt, and longs for its adult equivalent.

As the most social apes, we inhabit a mirror-world in which every important relationship, whether with spouse, friend or child, shapes the brain, which in turn shapes our relationships. Daniel J. Siegel and Allan N. Schore, colleagues at the University of California, Los Angeles, recently discussed groundbreaking work in the field at a conference on the school’s campus. It’s not that caregiving changes genes; it influences how the genes express themselves as the child grows. Dr. Siegel, a neuropsychiatrist, refers to the indelible sense of “feeling felt” that we learn as infants and seek in romantic love, a reciprocity that remodels the brain’s architecture and functions.

Does it also promote physical well-being? “Scientific studies of longevity, medical and mental health, happiness and even wisdom,” Dr. Siegel says, “point to supportive relationships as the most robust predictor of these positive attributes in our lives across the life span.”

The supportive part is crucial. Loving relationships alter the brain the most significantly.

Just consider how much learning happens when you choose a mate. Along with thrilling dependency comes glimpsing the world through another’s eyes; forsaking some habits and adopting others (good or bad); tasting new ideas, rituals, foods or landscapes; a slew of added friends and family; a tapestry of physical intimacy and affection; and many other catalysts, including a tornadic blast of attraction and attachment hormones — all of which revamp the brain.

When two people become a couple, the brain extends its idea of self to include the other; instead of the slender pronoun “I,” a plural self emerges who can borrow some of the other’s assets and strengths. The brain knows who we are. The immune system knows who we’re not, and it stores pieces of invaders as memory aids. Through lovemaking, or when we pass along a flu or a cold sore, we trade bits of identity with loved ones, and in time we become a sort of chimera. We don’t just get under a mate’s skin, we absorb him or her.

Ackerman ends Your Brain On Love with a touching personal coda. Her husband had a stroke, she tells us, that shattered his ability to speak. This book was informed by the way she observed her own survival process with her husband: their willful behavior as a couple, their careful preservation of their instinct to love each other, all of which made it possible for both of them to adjust to their new circumstances.

Many songs say “I can’t live without you”. Diane Ackerman’s article reminds us how often these words are literally true.