Just as Jessica Rabbit claimed, "It's not my fault, I'm just drawn this way," many of our actions in the dating world are direct results of our brain neurochemistry. As Louann Brizendine M.D. points out in her book "The Female Brain", "Though we live in the modern urban world, we inhabit bodies built to live in the wild." Much of this survivalist hardwiring remains encoded in our DNA even if we're stepping out of the cave in Manolo Blahniks. Although we are living in a liberated world where sex is not always tied to reproduction, many of these cues still govern to whom we are attracted and why.

"Is she hot? What does he do?"

While we criticize what we feel is a shallow culture where women are judged by their beauty and men by their money, there are actually biological bases for these preferences. Our brains have evolved over time to respond to clues to who would be the healthiest mate and those most likely to bear children (women) and those whose resources and staying power will help those children survive (men).

In a 1990 study, David Buss studied mating preference in 10,000 individuals in 37 countries around the world between the ages of 14 and 70. In every culture from Western Europe to Pygmies, women were less concerned with a potential husband's visual appeal and more interested in his material resources and social status (except for some regions in Spain). Men universally preferred physically attractive wives between 20 and 40.

Even if the older men weren't interested in having children with their younger spouses they still desired the women's appearance of fertility and women, even if they had the income capacity to support a child on their own, still wanted a man of equal or greater earning power. This difference in assessment can be shown through brain imaging where women in love show more activity in many areas of the brain while men show more activity in highly visual processing areas.

The Love Drug

Marianne J. Legato in her book "Why Men Never Remember and Women Never Forget" says, "What we think of the first flush of romantic love is in fact the result of a chemical cocktail, one that replicates many of the sensations of illegal drug use." In the first six to eight months of a relationship you are getting high doses of dopamine. When the new lover is away the brain is literally in a drug withdrawal state and you realize how much you miss it, giving credence to the old adage absence makes the heart grow fonder.

"Let's just cuddle"

Those who feel like the honeymoon is over may be reflecting the lowered levels of dopamine after time. However what replaces it is oxytocin and vasopressin which create feelings of calm and connection created by mutually pleasurable, positive experiences. oxytocin is what creates trust and when a woman says she wants to cuddle this is what is released in her brain. This changing chemistry is necessary so a couple can have room to focus the care on a child and not just each other.

Interestingly however men need to be touched 2 to 3 times more frequently than females to maintain the same level of oxytocin and will judge a woman to be no longer in love if she withdraws sex, while a woman will draw that conclusion if the man withdraws emotionally. The principal hub for both emotion and memory formation is the hippocampus which Brizendine finds is larger in the female brain whereas men have two and a half times the brain space devoted to sexual drive.

The many faces of Eve

When men talk about women being "hormonal" they are exactly right although they might not understand why. While male hormones remain fairly constant, the female levels of estrogen and progesterone change every day of the month. According to Louann Brizendine, some parts of the female brain change up to 25% each month. This fluctuation starts as early as three months and lasts through menopause so what is thought to be a "mood" is a biological reality.

What both genders share is the sexual desire trigger -- testosterone. Women actually produce it in their ovaries and adrenal glands and it peaks at ovulation as a sign that it is a good time for procreation. The second half of her cycle where progesterone is higher actually reverses the effect of testosterone so timing, as they say, is everything.

Not tonight honey, I have a headache

Guys have you ever taken your gal away for a long weekend to a romantic B&B or lovely resort and rediscovered your loving sex goddess. Is it something in the fresh baked blueberry muffins, the joy of sleeping in or the adult movies on demand? Actually it is the elimination of stress. Women's stress increases cortisol which blocks oxytocin and makes them not in the mood for sex while with men it is the opposite that after an intense physical challenge they crave sex even more. Cortisol also stays longer in a woman's body which explains when a man is wondering "What fight?" because he's already forgotten it, she is still steaming. Sex therapists find that women need a 24 hour conflict free zone with her lover to be in the mood while a man requires only three minutes for a turnaround time.

Beauty is not necessarily in the eye of the beholder

While some notions of beauty are culture bound such as giant lip rings in native African women or white powdered wigs for men in colonial England others are universal. In Nancy Etcoff's "Survival of the Prettiest" she points to studies where infants at three months actually gaze longer at attractive faces versus unattractive faces, proving that not all concepts of beauty are learned.

According to zoologist Desmond Morris, flawless skin is the most universally desired human feature and flowing, healthy hair is a close second. Much of this reflects on a woman's health. Skin is actually the largest organ of the body and smooth clear skin is just as much a turn on as bad skin is a turn off.

And ladies you are in luck because men are more attracted to a hip to waist ratio than pure thinness which actually makes no sense in primitive attraction because those ulta-skinny models do not appear to have enough body fat to conceive.

Let down your hair

In "A Natural History of Love" Diane Ackerman chronicles a history of men in love with women's long hair from Boticelli's nymphs to Bo Derek's braids. She notes, "A woman quickly learns that cutting her hair without warning her lover first is a bad mistake."

Modern day matchmakers encourage women to have long silky hair because that often tops the list of men's physical preferences in a mate. A lock of hair through history was thought to hold that person's magical power and indeed it does as hair is the chronicler of what we have eaten, what drugs we have taken, and how well we have cared for ourselves. A body struggling to fight off disease and famine does not have the extra energy to support a full head of long shiny hair. These tresses are thought to be so provocative that in certain cultures married women are supposed to cover their hair.

Tall, dark, and handsome

Size equals dominance in the animal kingdom. Aside from the fighting advantage of a tall man, his height also reflects that he enjoyed a good share of the resources and food supply as his body was developing. Nancy Etkoff studied taller men and found out that aside from holding an aesthetic advantage over their shorter counterparts they were perceived as being more powerful. Her research showed, "Men who are above average height (5'9" in the U.S.) acquire more resources than other men." In the 1900's the winner in the presidential election was always the taller candidate until 1968 when Richard Nixon beat George McGovern (and see where that got us), and only three percent of Fortune 500 company CEO's are five foot seven or less. Just as women of certain beauty are favored, so are taller men, and this perception becomes a self-fulfilling prophecy.

Luckily the differences of the sexes and how each approaches the mating game is a "hot" topic in medical and psychological research so if you really want to see how someone ticks, it's worthwhile to look not only at their body but at their brain.