Of all the qualities that give an attractive person an edge, here’s one you’ve likely overseen:  the limbal ring,  the dark circle around iris.  The limbal ring is the line that separates the colored part of the eye from the white (the sclera).   

It’s completely unconscious, the way we all judge others’ limbal rings. In the 20 milliseconds or so it takes to assess a person’s attractiveness, you’re factoring in the size and shade of the limbal rings. The bigger and blacker they are, the more attractive the eyes. People with the prettiest eyes have the most prominent limbal rings.  

This, anyhow, is the upshot of a recent study by Darren Peshek and his colleagues at the Department of Cognitive Science at the University of California at Irvine. The researchers showed volunteers eighty pairs of male and female faces.  Each pair of faces was identical except the eyes: one had dark limbal rings and the other had no limbal rings.  The volunteers were asked to pick which face was more attractive and to indicate their degree of preference.

Men thought women with the dark limbal rings were more attractive than those without, and women thought the same of men with dark limbal rings.  Men and women also judged faces of the same sex as more attractive when the limbal rings were large.

Looking into my baby daughter’s eyes, I see the blue of her iris is framed by a thick black limbal ring.  The contrast makes the white of her eyes so white they look blue. The very young have the thickest, darkest limbal rings.

Which is exactly the point.  The limbal ring serves as an honest signal of youth and health-desirable qualities, reproductively speaking. The ring fades with age and with medical problems.  It’s thickest from infancy through the early twenties.  A thick, dark limbal ring may make us appear younger.  It makes the whites of the eyes whiter.  This might be why so many people think light eyes are so sexy:  the limbal ring, when present, shows up more.  

There are so many ways to fake the appearance of youth. You can wear makeup and wigs and get tummy tucks, plastic surgery, Botox, and boob jobs.

But a fake limbal ring?

Yes, this too. Long ago, Japanese schoolgirls discovered the edge a limbal ring can give you by wearing “limbal ring” contact lenses. They make the eye look bigger and more defined. And while you’re eyeing these contacts, you might as well buy a set that expands your pupils too. Big, dark, dilated pupils signal emotional arousal. They, too, act on the unconscious favorably.

The limbal ring is well-named. Limbis means border or edge, and it’s related to limbic, meaning emotion or drives. The limbal ring, seen from inches away, is an intimacy zone. Don’t flirt until you see the whites of their eyes.

Why do men love breasts?

Let me count the theories:

1. Freudian (breasts remind men of their moms and the nurturing of childhood)
2. Evolutionary (breasts resemble buttocks, and prehuman ancestors always mounted from behind)
3. Reproductive (breasts are an indicator of age, and big breasts in particular are a marker of high estrogen levels, associated with fertility).

Do these reasons sufficiently explain why breasts are beloved — even in cultures that don’t eroticize them any more than the face?

If not, here’s another:

Breasts facilitate “pair-bonding” between couples. Men evolved to love breasts because women are likelier to have sex with — and/or become attached to — lovers who handle their breasts.

This idea came up in New York Times journalist John Tierney’s interview with Larry Young, a neuroscientist famous for his research on monogamy. According to Young, “[M]ore attention to breasts could help build long-term bonds through a ‘cocktail of ancient neuropeptides,’ like the oxytocin released during foreplay or orgasm.”

The same oxytocin circuit, he notes, is activated when a woman nurses her infant.

When women’s breasts are suckled, as they are during breastfeeding, the hormone oxytocin is released. Oxytocin makes the mother feel good and helps her bond with her baby. She feels loving and attached. The same reaction might happen if a man sucks and caresses a woman’s breasts during foreplay. In our ancestral past, the most titillated men may have been the ones to attract and retain mates and pass on their genes.

The “boobs-help-bonding” theory may not be the strongest explanation of why men love breasts, but it’s worth introducing to the debate. That said, there are many ladies out there for whom a lover’s suckling does nothing — and there are many breast-ogling boobs who know nothing of foreplay.

A few weeks ago, Israeli neuroscientists Shani Gelstein and Noam Sobel published a study about mind-control properties in human tears. The gist of the research, which enjoyed much media attention, is that women’s tears contain a chemical signal that reduces sexual desire in men. Tears were collected from the cheeks of emotionally-distraught women watching sad films and wiped on the upper lips of male volunteers. Compared to men who whiffed a salt solution control, the tear-sniffers not only had a reduced sex drive but also lower testosterone levels and reduced brain activity.

A leading explanation is that chemicals in tears generally reduce male aggression, making them more sympathetic.

How does this work? One theory is that one or more of the hundreds of chemicals in tears has “mind-control” properties, triggering specific predictable behaviors in others. (Here and here I write about how this happens in sweat, too.) One candidate is prolactin, a hormone associated with bonding. When inhaled in a person’s tears, prolactin may affect the sniffer’s hypothalamus, the part of the brain that produces hormones which in turn affect behavior.

Baby tears have not been the subject of a study yet (hopefully soon). But it’s not a far cry from certain that if there are chemicals in the tears of women that affect men, there are also chemical triggers in the tears of babies that affect their caregivers or anyone else that comes into contact with them. These tears may trigger care-giving instincts and reduce aggression toward the screaming infant.

I wonder: Infant abuse is relatively uncommon given how irritating a screaming baby can be. Are the people guilty of this crime more likely to be amnosiacs (loss of smell-sense) or have another form of brain damage that would prevent them from inhaling aggression-reducing signals in the baby’s tears?

Another theory: Kids cry all the time and sometimes it’s hard to tell when they really need attention. Might chemicals in emotional tears direct parents to respond appropriately when there is a real need for attention? Assume these chemical signals are only in emotional tears–not crocodile tears or sleepy-time tears. Do they help us intuitively know when it’s OK to let a child cry it out instead of rushing to soothe her?

Not long ago, people everywhere started to do the “finger game” on a first date. This is not as naughty as it sounds. As I describe in BLONDES, the finger game involves asking your companion for a look at his (or her) right hand. If his ring finger is longer than his index finger it’s a sign of prenatal exposure to high levels of testosterone. People with longer ring (than index) fingers are likelier to be more aggressive, better at sports, and more musically inclined. They may have more sex partners in life.

Now you can take the game to the next level: fingerprints.

Take a close look at the ridges on your companion’s fingers. (Actually, they’re best seen under a magnifying lens or photocopied and enlarged.) Most people have slightly more ridges on the fingers of one hand than the other.

More ridges on the right-hand fingers: This indicates higher levels of prenatal testosterone. He or she might master mental rotation – knowing which one of four abstract figures, revolved in three-dimensional mindspace, matches a diagram (a “masculine” task). Right-ridge dominant people are also better at aiming at a target and getting a bull’s eye.

More ridges on the left-hand fingers: This indicates lower levels of prenatal exposure. He or she may be a whiz at games like word associations, taking a word like clear and coming up with glass then Philip then opera then ghost, or naming as many round objects as she can in three minutes (considered “feminine” tasks). Compared to straight men, gay men have more ridges on their left pinkies and thumbs.

Four or more ridges on the fingers of one hand than the other: This reflects how much stress your companion weathered when he or she was a second-trimester fetus. For instance, researchers found that women who were 14-22 weeks pregnant when an epic ice storm hit Canada were more likely to have babies whose ridge counts varied greatly between hands. In nature, dramatic asymmetry is often a sign that the fetus has been stressed in some way. The more stress, the less symmetry. In fact, those with significantly asymmetric ridge counts between right and left hands were more likely to score lower in language and intellectual development as toddlers. Both fingerprint development and the brain may have been affected by constriction of blood flow to the placenta or stress hormone levels.

Other ridge count studies have also found interesting correlations: a significant difference between the ring and pinky fingers of the right hand is associated with less muscle mass in the lower extremities and a bulked-up upper body, including a thicker waist. A difference of around three or four more ridges between the thumb and pinky fingers is also associated with diabetes later in life. Asymmetries are also connected to cleft lip, dyslexia, schizophrenia, infections, and other prenatal problems.

Around ten weeks after conception is when the bottom (basal) layer of fetal skin outgrows the top (epidermis), and the tension between the two causes the skin to buckle. At this time fingerprints are like wet cement: any disturbance until mid- pregnancy may leave a lifelong impression. At this time the skin and the brain are both are made of the same raw material — fetal ectodermal tissue. Any disruptive event in the womb left its mark on both. This means that fingerprints give us clues about the brain.

You would like to know more about the minds of the people you date, which is why you’re analyzing their fingers. Of course if you could read their minds, you’d know they think you’re crazy.

This bit of science arcanum is especially cringe-worthy.

Many years ago, scientists first discovered that a large minority of women have Y-chromosome gene sequences in their blood. At first glance, this seems strange. Men are born with Y-chromosomes but most women are not. The male cells in these women must’ve come from somewhere else.

But where?

The most obvious source is a fetus. Nearly every woman who has ever been pregnant or had a baby has cells from her fetus circulating in her bloodstream. These cells filter through the placenta and reside in the mother’s bloodstream and/or organs — including her heart and brain — for the rest of her life. This condition is called microchimerism, named after the Greek chimera, a creature composed of the parts of multiple animals. Pregnancy-related microchimerism explains why women with sons would have Y-chromosome sequences in their blood.

This is fascinating enough. But how do you explain why women without sons also have male cells circulating in their bloodstream?

This was the subject of a study by immunologists at the Fred Hutchinson Cancer Center. They took blood samples from 120 women without sons and found that 21 percent of them had male DNA. Women were then categorized into four groups according to pregnancy history: women with daughters only, spontaneous abortions, induced abortions, and no children/no abortions.

While the number of women bearing male DNA was highest in the groups that had abortions (nearly 80 percent), women who had only girls or no babies (20 percent) also had male cells in their blood. For no apparent reason.

There are other reasons why women in the fourth group carried male cells: inherited in the womb from a male twin that passed, from a miscarriage they did not know about, from their mother via an older brother…

Or through sexual intercourse.

There remains a possibility, however remote, that cells from a lover may pass be transmitted during sex. Those cells may hang out forever in the recipient’s body, taking residence in any organ. These cells are the imprint of lovers past, a trace of living history.

Might a woman’s bodily fluids enter a cut in a man’s genitals as well? Could men carry around the genes of women they’ve slept with?

The imagination is stirred. What are those foreign cells doing in hearts and minds? Are they wreaking havoc in our heads? Do the cells of former lovers clash? In a science fiction scenario a person could even take a drop of her own blood, isolate a cell from her former boyfriend, and clone him. Then do with him what she will.

The upshot of this research? It’s yet another reason to use a condom.