If advice websites are any indication, women around the world wonder why their boyfriends check out other girls. Sites like Dearcupid.org, Answers.yahoo.com and Blurtit.com, to name just a few, field these questions and provide pages and pages of reasons why both men and women look.

These responses come from real people. Some of their answers are part of a bigger, evolutionary truth, and some try to explain away social behavior. Here’s what the experts had to say.

“The answer is really simple. Truth is we just can’t help it.” - A male reader, April 29, 2008

Sure, both men and women look. But men are driven to look by an evolutionary mechanism that Robert Trivers explains in Parental Investment Theory, which predicts which sex will be the fast, indiscriminate mating sex and which sex will be the more discriminating sex.

Because women must spend a minimum of nine months pregnant and several years breast feeding a child, women are more careful when choosing a partner, said Doctor of Evolutionary Psychology Aaron Goetz, an associate professor in Cal State Fullerton’s psychology department.

Biologically, men can have sex with as many women as they want without having to worry about all the work.

“Why might men be more interested in casual sex and be the dogs and be the consumers of pornography and be the patrons of prostitutes and all that sort of stuff? It makes sense why men do that because in the past that psychology would have been selective for them,” Goetz said.

Sue Passalacqua, a marriage family therapist and the associate director of the Cal State Fullerton’s Women’s Center, agrees that women look just as much as men, but she thinks it is a learned behavior.

“We are socialized differently. Because of that, we might see more men overtly checking other women out because it’s socially acceptable. That’s a masculine thing to do,” she said.

For modern man to seek out many partners does not serve society. Somewhere along the way, our ancestors realized that monogamy was the best way to ensure our genes made it into the future. One of the main benefits of monogamy is reduced parental uncertainty: our male ancestors could keep better track of which offspring were their own rather than putting energy into raising another man’s child.

Though humans have evolved into monogamous creatures that don’t need to look around for other mates, “checking out” still happens.

“Honestly, it’s ridiculous to get upset by something he is genetically programmed to do,” Samantha von Sperling wrote, in an e-mail. She is the director of Polished Social Image Consultants, an image consulting and dating firm in Manhattan, NY.

Another reason our ancestors looked around was to assess mate value. According to Goetz, a man’s value as a mate increases if he can offer a woman monogamy.

“[Ancestral man] can acquire a better mate with a higher mate value, because he’s saying, ‘I’m sacrificing all of these wives for just you, for just one, who’s it going to be.’,” Goetz said.

Before love, commitment and respect became involved, our ancestors just looked for the best they could do.

“If my partner is of lower mate value than me, it would be evolutionary beneficial for me to seek attractive alternatives,” Goetz said. “That mechanism would have been selected and favored over one that said ‘love your partner, regardless, no matter what’.”

Goetz explained that if a female were mated to a male who experienced an injury or something else that disrupted his ability to provide, it would be beneficial for her to find someone with a higher value.

This is still evident today, in that one of the best predictors of divorce is when a man loses his job, and is no longer able to provide for the family. Though people cite other reasons for the breakup, Goetz said the research shows that women leave men who can’t pay the bills.

Our ancestors also figured out that bi-parental care increases the chance of offspring survival more than just single parenthood.

All the while, love evolved, and people began to fall in love to ensure that they would stay together and get their genes into the future, Goetz said.

“If a man loves, respects and honors the woman he is with, he wouldn’t have any interest in someone else no matter if she is wearing a tight top or not.” - A female reader, Oct. 27, 2008

According to Goetz, there are lower-level, unconscious things men and women do to maintain a relationship and keep from “checking out” others.

He explained an experiment that studied reaction times and memory of attractive faces. The two types of subjects were people in the initial, infatuation stage of a relationship, and people who have been “love primed,” meaning they were asked to write a letter to their partner or to think about their partner. The subjects were shown an array of faces, for a few seconds each, and then shown the faces again and asked which ones they remember. The men and women who were infatuated or love primed did not remember the attractive faces.

“People have interpreted this as, wow, you’re not even [seeing the attractive person],” he said.

Another test involves attention to attractive faces. Goetz describe “attentional adhesion,” which means that some stimuli are so attractive that they are sticky, and it is difficult for people to look away. But when love primed, people can just as easily look away from attractive faces as unattractive ones.

“We’re talking fractions of a second here – 300 milliseconds – these things are happening, and people are thinking, I don’t care about the attractive people. So it’s even deeper down that we have this nice relationship psychology,” Goetz said. Ideally, couples in a relationship could love prime themselves all the time. “That would be a good way to reduce infidelity.”

While Goetz does actual research, Passalacqua deals with the more practical applications of the topic. Passalacqua, who has a Master’s Degree in counseling and her own offices in Fullerton and Whittier, Calif., outside of her duties in Cal State Fullerton’s Women’s Center, said 85 percent of the issues her students bring her are relationship related.

One of the typical issues she deals with both at Cal State Fullerton and in her private practice is infidelity.

“Infidelity can defined in so many different ways. There are different levels,” she said. “That’s a level. Feeling like you’re being betrayed because he’s checking out other women.”

This can cause problems since most men feel that as long as they don’t have sex with another woman, they haven’t done anything wrong.

According to Passalacqua, any time a third person is brought into the relationship, what she calls triangulating, it can be a symptom of a larger issue of intimacy. He or she may have trouble committing and being intimate with one person because they fear they aren’t going to be accepted for who they are.

“So instead, what they do is stay more surface with people, and what that might look like with men is that they get involved with more than one woman, because being intimate with just one person is uncomfortable for them, unsafe for them.”

Passalacqua said intimacy is what keeps a relationship going, and that both men and women strive for an intimate, healthy relationship.

“I believe deep inside that men and women want the same things, we’ve just socialized men and women so different that we go about getting it in very different ways,” she said.

Goetz agrees, citing that over 90 percent of people in all societies marry, so settling down is a goal for most.

“Guess what? To your surprise, there are actually gentlemen out there who consciously choose to not look while out with their woman because they actually care about what hurts their woman.” - A female reader, May 6, 2008

“Men do have to look,” wrote Dennis Neder, president of Remington Publications, in an e-mail. “We can control it, but it simply creates stress in us since the behavior is pre-programmed. People are always able to go against these sorts of reactions – to fight against them, really – but do so with great accompanying stress.”

Remington Publications hosts the weekly “Being a Man” Radio on BeingAMan.tv.

Goetz agreed that people can control it to an extent, and thinks that they should.

“Let’s say you’re not in the very beginnings of a relationship and you aren’t love primed,” Goetz said. “Then you might see an attractive person walking down the street, and you think, oh, they’re attractive. That’s the part you can’t necessarily help. But what you can help is you walking up to that person and flirting with them and accepting their number.”

Goetz made a distinction between low-level things, like the initial look – the one our ancestors did to check out whether the other is friend or foe, or to initially assess mate value – and actually crossing the line. He said you can stop the look there.

“Because it would hurt the other partners’ feelings,” he said. “You can inflict significant damage when you play with your partner’s emotions.”

Goetz also questions whether it’s even right to look.

“There are a lot of things that humans can’t help that are unacceptable,” he said. “People murder and rape and do all these sort of things every single day and they’re bad things, but we don’t just say, oh, because we do it, we can’t help it.”

Goetz cited the is/ought fallacy, or Humes law, which argues that just because something is a certain way, that doesn’t mean it is right.

“You’re committing an error in logic if you’re thinking that just because something is some way, it ought to be some way,” he said.

But again, Passalacqua is more practical. She thinks everyone looks, and doesn’t believe “checking out” is a deal breaker. It is not a sign that the lookers don’t love their partners.

“I think a man can be very satisfied with his woman and in his relationship, but look. We don’t live in a bubble,” she said.

Doctor Kelley Brigman, author of Marriage – A Simple Guide to Success, agreed.

“I don’t think men or women will ever quit being aware of others around them, but it is entirely possible to look and go on,” she wrote in an e-mail.

Passalacqua added that a man might be motivated not to look out of respect for his partner. But women should try to see where their insecurities are coming from and address them so that they can decide that it doesn’t bother them as much, she said.

“Look at how much hub bub this question generated. Gentleman out there, can you imagine what kind of rise we would get if they knew what we were actually thinking while our eyes lingered just a bit too long.” - A male reader, Feb. 6, 2008

It’s impossible to figure out where each person’s look falls – just a look or thoughts about sex? And both Goetz and Passalacqua said it’s impossible to know who’s telling the truth.

Is there hope that men will never look? It’s hard to know who to believe.