Let’s begin this month’s column with a thought experiment. I’ll give you four words, then you take a minute to free associate to these words. Let any images or thoughts come into your mind.Here we go: WOMEN, MEN, SEX and RELATIONSHIPS.What do you have? I suspect that most of us started to think about widely-held sex and gender stereotypes within relationships.  Perhaps you thought about a seriously amorous guy constantly trying to convince his wife or girlfriend to have more sex with him. Perhaps you thought about who initiates sex more in your relationship.MORE: Savor Sex With All Five SensesOr, maybe you thought longingly about how you used to have more sex with your partner, but he just doesn’t seem that interested anymore.Despite what our gender stereotypes tell us (amorous man, headachy woman), I’ve spoken with many women who have told me that it’s their male partners who have lost steam in the bedroom. It’s true: men can lose interest in sex, too.Observing a change in frequency of sex can leave many women feeling like a failure for not arousing their man’s true horny self. It can prompt troubling self-doubt: Is he attracted to me? Does he still find me sexy? Is he having an affair? Why is he looking at pornography when he can have his way with me?In trying to sort through all of this, I turned to the research literature to help find out what’s “normal” in terms of changes in men and women’s sexual desire within a relationship. Turns out, this is not an easy question to answer. But there are a few trends that keep cropping up:Men want sex more. Consistent with the commonly-held stereotype, men, on average, have more desire for the physical act of sex than women. Men think about sex more than women do and are more likely than women to emphasize the importance of the physical act of sex relative to the emotional side of sex. In short, men have far more permissive sexual standards than women, and this is relatively well-known.Assumptions lie. These average differences between genders mask substantial variability within each gender. Said differently, the sexual desire of any two women is likely to be more different than the sexual desire between any man and woman. This fact suggests that our stereotypes about gender and sex are mostly wrong.There’s no such thing as a “normal” amount of sex. There’s a huge range in the amount that most couples have sex on a weekly basis. One recent report indicated that married couples in their 20s and 30s tend to have sex on average about 12 times per month (with about 68 percent of the sample falling between three and 21 times). Married couples in their 30s and 40s are having significantly less sex, averaging about six times per month (with about 68 percent of the sample falling between zero and 13 times). In short, there’s quite a big range in how often couples get it on, which means spending any time thinking about “what most people are doing” is probably a useless exercise.COLUMN: Do You Need Couple’s Therapy?This last point illustrates a key theme I want to emphasize. Don’t think about and derive your sexual satisfaction from what you think other people are doing in the bedroom. Instead, think about what you want and what feels right for you and your partner.Approaching the ProblemWhat women need to remember is: It’s not always about you! I don’t mean this to be insulting (if you’re unhappy, of course it’s about you); I mean it as an invitation. Speak to your partner about your wants, needs and desires before you start fretting about what you might be doing wrong. There’s probably more going on than you realize.For most people, men and women alike, there’s a natural decline in sexual activity once a relationship becomes established. Add to this a stressful job, exercise and maybe kids and, well, you start mixing the ingredients of chastity. If your man has lost some of his mojo, don’t be shy. Ask him directly what’s up.I know what you may be thinking: How can I talk directly about this? First, describe what the problem is. Don’t blame, just describe. Second, express how you feel (again, without blaming). Third, inquire about what’s going on with your partner in a way that invites further discussion. Finally, suggest a plan. You don’t have to shoulder the burden of initiating sex all the time, but you can suggest a plan for more action.Before ending this column, I’d like to put forth one final thought: sex begets sex. It’s like babies and sleep. To encourage good sleeping in children, you keep as regular a schedule as possible and you make sure they get their naps on time. The same goes for sex. The more you do it, the more you will do it. The best way to interrupt a dry spell is to not think about the dry spell. Just do it. Then, do it again.QUIZ: Are You Sexually Satisfied?