Remember the Seinfeld episode where Jerry’s penis has an argument with his brain and loses? It’s a classic: We’ve all been torn between love/lust and logic. (If you haven't, check for a belly button because this isn’t your home world.) This brilliant bit of comedy is totally relatable but a little misleading in one way: The brain is the one that sends signals to the penis in the first place. It’s pretty reliably running things, IMing the other body parts like crazy, regulating chemicals, making calculations and responding to stimuli, half the time without you even knowing about it. There’s a reason “the brains of the outfit” denotes someone who is really in charge.

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So why does the brain sometimes signal us to do stupid things, especially in regard to sex and relationships? Isn’t that a little like one conjoined twin punching the other in the mouth? How does the brain decide who attracts us? What is it doing behind our backs, and how do we change as we mature?

Here are some of the ways the joys, quandaries and mechanics of sex are all in your head.

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1. Size matters.

The preoptic area of the hypothalamus, which regulates mating behavior, is a little more than twice as big in men as it is in women and has twice as many cells. Medical Net says difference starts to show up when we’re about 4 years old.

2. Location. Location. Location.

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The male brain devotes twice as much real estate to sex as the female brain. They think about it, but do they listen about it?

Some men certainly do, and maybe more will now that there’s visible evidence of what some women have been trying to tell them for quite some time: there is a great deal of difference between vaginal and clitoral stimulation. Now you can see it. Researchers at Rutgers University used an MRI to map what locations on the sensory cortex correspond to the vagina, clitoris and nipples. All three clearly in very different locations in the brain.

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The fact that nipple stimulation lit up the areas corresponding to the genitals as well as the chest area seemed to come as a surprise. Linda Geddes of New Scientist quotes researcher Barry Komisaruk as saying, "When I tell my male neuroscientist colleagues about this, they say: 'Wow, that's an exception to the classical homunculus,'" he says. "But when I tell the women they say: 'Well, yeah?' It may help explain why a lot of women claim that nipple stimulation is erotic, he adds.”

Claim?

Anyway, those images are pretty spiffy, and they're ripe for a new line of greeting cards. Valentine's Day is coming and those hearts and cherubs need a break.

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3. In what part of the brain do we find the “Not tonight, honey" headache?

It was a comic trope of the olden days for women who didn’t want sex to opt out due to headache, but it turns out migraine sufferers seem to have a higher libido than other sufferers. LiveScience reported on a 2006 study from the Wake Forest School of Medicine that found that people who suffer from migraine headaches reported a sex drive about 20 percent higher than those prone to regular tension headaches. The key might be the neurotransmitter serotonin. High serotonin levels are associated with low libido; the researchers reported that migraine sufferers had low serotonin levels. And About.com reports that a 2001 survey of women who had sex during migraines 30 percent noticed a decrease in pain, 5.3 percent said the pain increased, and 17.5 percent reported the pain went away.

It’s hard to believe anyone can have sex during a migraine, but for that 17.5 percent it sounds like the traditional “Go to bed,” advice worked out pretty nicely (even if "and get some rest" wasn't part of it).

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4. Considering all the kinds of headaches it causes rather than cures, why do we even have sex in the first place?

Dr. Joseph Shrand is an instructor in psychiatry at Harvard Medical School. (He also happens to be Joe from the TV show "Zoom!" -- if you’re a '70s kid that brought on some of the feel-good chemistry we’re going to talk about in a minute.) Shrand has a wonderfully concise way of explaining why we let ourselves in for the sturm und drang of sex. When we describe love and sexual passion “we use the word 'intoxicating' and that’s a very important word,” Shrand said in a phone interview. “There is dopamine involved in that sort of lustful attraction,” a neurotransmitter associated with excitement, reward, desire, pleasure and in some case addiction.

“When we are falling in love with someone all we can think about is that person ... it’s a remarkable, remarkable feeling and it’s a pleasure. There’s huge biological significance to that. If we didn’t feel pleasure when we have sex we wouldn’t have babies. I mean, can you imagine if sex was really uncomfortable and horrible and not reinforcing? Why would you do it?”

Search me.

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“The orgasm is pleasurable as a way of saying 'We want to do this again!' You want to do this as often as you can and if you don’t have somebody to do it with you’ll figure out how to do it anyway,” all of which he says is adaptive because it’s how we get our genes into the next generation.

The trick, Shrand says, is getting your limbic system, an ancient part of your brain that is the seat of those primal drives and emotions, to work with your “new brain,” the more evolved neocortex that helps you consider causes and consequences. “You have to be able to shift gears in your prefrontal cortex and make a plan,” a plan to get that person, to keep that person, to understand the consequences of what you’re doing, which isn’t easy when your brain is “overwhelmed by dopamine and lust,” he says. “It’s amazing we have relationships at all.”

But dopamine alone won’t bind you to someone. That’s the province of oxytocin, which has been called “the cuddle hormone." Oxytocin creates a feeling of warmth, security, bonding and trust. “It’s a much deeper, more powerful and more modern part of love,” Shrand says, “because oxytocin is a much more complicated chemical which implies it’s really relatively more recent than [a simple one like] dopamine.” So when you find yourself in the grip of lust versus logic, that may well be the primal brain and brain chemistry arguing with the more modern parts. It’s real and we all go through it. Hopefully that raised your oxytocin enough to feel a little better about the whole crazy mess.

5. The downside to bonding: Getting stuck.

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If our brains are so smart why do they let us attach to people who might not be so great for us? Catherine Salmon, an associate professor of psychology at the University of Redlands, says the answer might have something to do with oxytocin, that very same chemical that creates those feelings of warmth and security. It sounds great, right? So how could something so good make us stick with people who aren’t?

“Oxytocin is not only released during birth and breast-feeding but also during orgasm,” Salmon wrote via email. “As a result, you feel more attached to the guy who you shared that orgasm with, which is great if he's a good mate choice but maybe not so much if you're Rhianna and he's Chris Brown. He may be a good lover but poor dad material, and yet you'll be attached to him and perhaps stick with the relationship longer than you should.”

So if you hear Dionne Warwick when he walks into the room and your friends -- who didn’t share an oxytocin moment with him -- hiss like wet cats, you may be seeing the situation through oxytocin-colored glasses (I have so many if I buy another pair I get the next one free).

6. There’s a reason people shout, “Oh, God!”

Sexual relationships are powerful enough to feel like spiritual experiences and there may well be a good reason for that. From "The Scientific American Book of Love, Sex and the Brain" by Judith Horstman and Scientific American:

“Jefferson University neuroscientist Andrew Newberg scanned the brains of praying Catholic nuns and meditating Buddhist monks and found some overlap between their neural activity and that of sexually aroused subjects (as seen in scans from other researchers). The correlation makes sense, according to Newberg. Just as sex involves a rhythmic activity so do religous practices such as chanting, dancing and repetition of a mantra. Religous experiences produce sensations of bliss, transcendence beyond one’s self and unity with the loved one that is very like the ecstasy of orgasm. That may be why some mystics, such as St. Teresa, describe their rapture with romantic or even sexual language.”

The book also notes that another study found that thinking about God and religion goes on in various parts of the brain, including the same areas we use to think about mundane experiences. No single "God spot" has been pinpointed in the brain. So you'll have to settle for the other G spot a little lower down.

7. Testosterone: The puppet master.

Speaking of heavenly, let’s talk about male anatomy for a moment. Wonderful though it is, it has its share of headaches and this certainly sounds like one. Oxytocin may keep us attached in ways we normally wouldn’t want to be, but testosterone has men doing things they don’t even know they’re doing, like getting erections. According to Louann Brizendine in "The Male Brain," “These reflexive erections are different from true sexual arousal because they come from unconscious signals from his spinal cord and brain, not from a conscious desire to have sex. The testosterone receptors that live on the nerve cells in a man’s spinal cord, testicles, penis, and brain are what activate his entire sexual network. Women are surprised that the penis can operate on autopilot and even more surprised that men don’t always know when they’re getting an erection ... We women often notice the rising tide before he does.”

We notice because we care.

8. So testosterone is pretty powerful; does it ever relax its kung-fu grip on a guy?

Indeed it does. Marlene Zuk, professor of ecology, devolution and behavior at the University of Minnesota, notes in her forthcoming book "Paleofantasy: What Evolution Really Tells Us About Sex, Diet and How We Live" that “conventional wisdom holds that men are unreliable long-term mates” because they’re always questing for new partners, but “what if the urge to find a new mate is ameliorated by the experience of fatherhood itself?”

In a long-term study of 600 men in the Philippines, anthropologist Lee Gettler of Northwestern University measured the men’s testosterone and predicted those with higher testosterone levels at the start of the study would become “partnered fathers” by the follow-up, four and a half years later. And he was right.

“But then something interesting happened,” Zuk writes. “The fathers showed a dramatic decline in testosterone compared with both their own single, pre-paternal levels, as well as the levels of the men who had remained single. What is more, testosterone was lowest in those men who spent at least three hours a day caring for their son or daughter, after controlling for the effects of sleep loss and other variables.”

“This study is illuminating for several reasons,” Zuk writes. First off, the same men being re-measured, instead of fathers being compared to single men, allows for fewer variables. Second “it indicates a finely tuned back-and-forth between a person’s physiology and behavior. Cues from the environment can influence fathers’ hormone levels as well as those of mothers. The scientists suggest that while seeking a mate requires characteristics that may be antithetical to being a good father, it is, in fact, possible to have it all, and testosterone acts as the mediator.”

Finally, Zuk writes "As Peter Gray, an anthropologist at the University of Nevada, pointed out in a commentary accompanying the article, the research serves as a nice case study of the relevance of evolution to everyday human life. The trade-off between mating and parenting is one that is predicted by evolutionary theory, and it means that a longing for new sexual partners might not be part of our heritage.”

9. And you thought your new iPhone was fast.

In 2008, doctors Stephanie Ortigue and Francesco Bianchi-Demicheli found that it took .02 seconds for the brain to register a person in a swimsuit photo as desirable or undesirable. At that speed you’d think it would go “zip!” from the parts that process visuals to the higher parts that make decisions. It did go that way sometimes, but sometimes those higher functioning parts started responding very early, writes Carl Zimmer in Discover magazine. Those higher parts, that handle self-awareness and empathy, might be instructing the eyes on who is attractive and telling the emotional centers how to feel about them.

Yep: the most important decision of your life might have started as a Quick Pick.

10. So my brain looks, debates and decides, like on "American Idol"?

Right down to the three conferring panelists.

Time magazine’s Maia Salavitz reports that in a study done on speed dating by Trinity College in Ireland, male and female subjects were hooked up to an MRI machine and asked to judge potential candidates by photograph before the 5-minute speed-date meeting. The people they thought they’d like and the people they actually asked out matched up 63 percent of the time to the people they wanted to ask out after the five-minute date, writes Stephanie Pappas of LiveScience and researchers found out what part of the brains were fired up during that initial decision-making process.

First, Salavitz reports, we have a twofer, the dorsomedial prefrontal cortex, which contains two sub-regions: one that judges attractiveness, the other that judges what’s attractive to you, though not necessarily to everyone else (that’s the restromedial prefrontal cortex or rmPFC). The first is what tells me “Ryan Gosling is handsome,” and the second tells me “But I still prefer Benicio del Toro.”

The people who got the most positive response aroused the ventromedial prefrontal cortex, “an area that has previously been found to react to appealing faces,” writes Salavitz, but that didn’t mean those people got asked out.

Maybe that’s the rmPFC butting in and saying, “Just cuz she’s cute doesn’t mean she’s for you."

And that’s why they have three judges on "American Idol."

These are just a few of the ways your brain is trying to guide you through the awesomely complicated world of lust and romance. It might not look like the sexiest body part, but it really is. Which body part do you think came up with Love to Love You Baby?