Last month’s column focused on the nature and meaning of the gender-related differences that scientists have identified. This column will explore the fuzzy line that both separates and unites the concepts of male and female, and especially as they relate to bonding behaviors.

Human reproduction requires separate M/F sexual systems, but also a mutual willingness to engage in sexual behavior, and to bond if pregnancy results. Since child-rearing involves an extended parental commitment, the impetus to engage in sexual behavior must be innate and strong. Call it love—an intense joyful attraction between two people that’s typically associated with romantic and familial bonding.

We’ve long associated love with our heart, but it really encompasses our entire body and brain, our entire psychological being, our entire lifespan—and researchers are now unlocking many of its mysteries. Romantic love appears to follow a three-stage lust-attraction-attachment sequence that emerged out of our strong biological drive to reproduce, and our need to nurture children over their extended developmental period. Couples in love who have no plans for children apparently also follow the same biological/psychological sequence.

Scientists can now use brain-imaging technology to identify the specific brain and chemical systems that drive the complete process. The hypothalamus, caudate nucleus, nucleus accumbens, septum, and several frontal lobe areas are especially active when love is on our mind—and dopamine, norepinephrine, serotonin, endorphin, oxytocin, vasopressin, testosterone, and estrogen are the molecules that seemingly jump for joy as they excite and inhibit the body/brain systems that regulate the behavior of love. So it’s party time all over our body and brain when love is in the air.

Lust is a somewhat abstract process that initially emerges during adolescence. Testosterone and estrogen provide an encouraging push to just go looking. Pheromones and personality predispositions probably assist in narrowing the field (such as to eliminate close relatives). Further, variations in the systems and molecules that regulate sexual behavior may bias a person’s focus to a same sex rather than to an opposite sex partner. Sometimes skyrockets go off immediately, and sometimes things heat up slowly—but at some point generalized biological lust leads to focused psychological attraction.

Attraction is highly focused attention. One’s beloved becomes foreground and absolutely everything else becomes background. This is a time for fine-tuning initial impressions, and so the couple spends many hours checking out each other. If the attraction remains mutually supportive long enough to resolve any nagging concerns, the third stage of attachment kicks in.

Attachment is for the long haul, since it must maintain the relationship through inevitable distractions. Oxytocin, vasopressin, and endorphins become part of the bonding glue that maintains the relationship whenever temporary troubles arise. Wedding promises before family and friends, joint ownership of possessions, and children who need parental love and nurture often provide a needed cultural bandage while a marriage hurt heals.

Children typically enter into a loving relationship with their parents at the attraction stage—the soulful gazing into a parental face, the oxytocin boost from nursing, the cuddling behavior that elevates oxytocin, vasopressin, and endorphin levels in both parents and child. Survival depends on children falling in love with their parents and their parents falling in love with them. Mother Nature sees to it that they typically can hardly resist each other—until puberty, when lust kicks in and adolescents have to go looking elsewhere to help continue their species’ biological destiny.

Love is obviously more complex than this, and it encompasses many contentious issues of appropriateness and legality that currently confront our culture. Some folks think that reducing love to neurochemicals and activated brain systems demeans the concept of love, but I think not. How wonderful to realize that falling and staying in love involves a perhaps forever-mystical synchronization of two complete biological organisms—and that’s what the poets and song writers have basically told us all along.

The biochemical complexities of love allow people who are infertile to still fall in love and yearn to rear children, and people who have lost a partner through death or divorce to fall in love again. People are willing to adopt, teach, nurse, and coach the children of others. Two people who have few friends and seemingly no prospects will suddenly discover each other. Love is such a marvelously supportive and adaptable human property! A pity when its expression disappears during personal and cultural conflicts.