The best way to think about sexual arousal is the Dual Control Model of Sexual Response, a newish theory developed at the Kinsey Institute by Erick Janssen and John Bancroft. As you might guess from the name, the dual control modelhas two parts:

Sexual Excitation System (SES) – SES is the gas pedal of your sexuality, responding to sexually relevant stimuli in the environment, from visual stimuli to tactile stimuli and everything in between. It constantly scans the environment (including your own thoughts and feelings) for things that are sexually appealing and as it finds things, it sends signals from the brain to the genitals to tell them, “Turn on!” SES is constantly at work, on an utterly subconscious level.

What constitutes a “sexually relevant stimulus”? Some things, like the sight of a woman’s genitals, might be innately sexy, but many other things are learned. This might be part of the reason why men vary in their sexual preferences. (This doesn’t seem to apply to gender preference – there’s mounting evidence that sexual orientation has some genetic basis and that boys establish their sexual orientation very early.) It also helps explain how “what’s sexy” has changed over time – culture shapes our understanding of what’s “desirable.” For example, modern American culture tells men that mid-adolescent bodies, with their relatively narrow hips, low body fat percentage, and young faces (with tanned complexions among Caucasians), are the most attractive. In paintings from the Renaissance, many of the women have rounder, softer, more developed bodies and porcelain skin. And the difference makes a kind of sense – during the Renaissance, the only women who had leisure and luxury enough to gain weight and stay out of the sun were the wealthiest women. Only the wealthiest women survived into their thirties. Nowadays, the wealthiest women have the leisure and luxury to have personal trainers, personal chefs, and tanning beds.

Sexual Inhibition System (SIS) – SIS is the brakes system. Research so far shows that it’s likely there are two different SIS systems, one that responds to fear of performance failure (erectile dysfunction, premature ejaculation, etc) – call that SIS-1 – and another that responds to fear of performance consequences (STI transmission, unwanted pregnancy, social consequences) – call that SIS-2. Just as SES scans the environment for turn-ons, SIS scans from turn-offs, and all day long it sends a steady stream of “Turn off!” messages to your genitals. SIS is responsible for keeping a person from getting inappropriately aroused in the middle of a business meeting or dinner with your parents. SIS is also the system that slams on the brakes if, in the middle of some nookie, your grandmother walks in the room.

A lot of what constitutes a “threat” depends on the context. The extent to which a person’s brakes are engaged because of fear of an STI changes depending on the perceived likelihood and the perceived impact of that STI. Using a condom? Know your partner’s health history and sexual history? Trust that you’re both being monogamous? Less threat. No condom? No history? Potential for non-monogamy? More threat. Same with social consequences: damage to your social status, your reputation, to your relationship, all can serve as threats, depending on how likely they seem and how negative they would be if they happened.

The Dual Control Model is “the man behind the curtain” – it’s the mechanism that underlies arousal. Arousal is really two processes: providing gradually increasing stimulation for the SES, and getting rid of everything SIS might respond to, which includes physical, emotional, and social risks. Both men and woman have both of these systems, and a person’s level of sexual arousal is a combination of how much stimulation SES has gotten and how little stimulation SIS has gotten.

If you had to guess, would you say men or women tend to have more sensitive SES? Yeah, men. And which has more SIS? Women. More sensitive gas pedal (SES) and less sensitive brakes (SIS) means easier acceleration. Dig? This varies greatly from person to person, but overall, men tend to be easier to arouse than women.