Feb. 13, 2009 -- With the approach of Valentine's Day, love gurus, relationship mavens and infidelity experts (is that the same as an infidelity researcher, or is it someone who cheats on their spouse without getting caught?) take to the airwaves to tell you how to evaluate your relationship and determine the faithfulness of your partner.
While human relationships have existed for countless generations, biological research on what makes them happen has been around for a much shorter time.
The past few years have yielded a great deal of information on what makes men and women get together -- or stay apart.
First, however, a warning: A lot of the biology behind human relationships developed in eras when mating was much different than it was now, so natural instinct may not match the best decisions.
While people may once have been most worried about passing on their genes, they may need to be more worried with keeping their spouses happy now.
So while many of these nuances of human relationships may seem to justify infidelity, they might also explain why you have feelings that don't mesh with what you know to be best here in the 21st century.
As we take a look at five ways your body gives you relationship signals: don't worry too much if your ideas on relationships and the ideas here don't completely mesh. Genetics aren't destiny in life when it comes to disease, and they aren't destiny in relationships either.
As Women Cycle, Their Hormones May Give Signals to Cheat
Martie Haselton, a psychology researcher at the University of California Los Angeles, has done a number of studies on how ovulation affects women's behavior.
It seems that right before they are most likely to get pregnant, women with less sexually attractive partners find themselves tempted to cheat on their partners.
"We don't have unfaithful behavior in our studies because the frequency of actual cheating is pretty low," said Haselton. "What we asked them is who they notice, who they flirt with, who they are attracted to, and who their sexual desire is actually focused on."
What her research and other research in the area has shown, said Haselton, is that while we may look at how females in other species go through heat when they are prepared to mate, "there is something similar to that going on in humans."
The reason for this, she explains, is that there is a high cost in carrying a child for nine months and then breastfeeding them, and so a woman in earlier times would have wanted to ensure that the child had the best genetic material possible.
"A woman had only so many offspring and it was important for her to choose the best genetic father for all of her offspring," Haselton said.
She said her hypothesis may seem controversial because it implies that unattractive men are not good choices as mating partners. But she notes that changing times have made what is important in a partner change, and evolution in desires hasn't necessarily kept up.
"When cast in that light, this kind of information can be empowering to women as they navigate…through these situations," said Haselton.
"What matters most in the modern environment is not just basic survival, because most people have that down, but quality of emotional life."
She noted that a man who will help raise children is probably more important for ultimate happiness than if mom had strayed with a more attractive neighbor.
"It's not saying that women should cheat, or that that's a better way to live one's life," Haselton said of her research, adding that values are independent of the science.
"It's important to know where [these feelings] come from, so that one can make a fully informed decision about whether to follow them. When women experience these desires…that doesn't bear at all on whether they love their long-term partner."
"We make decisions not to follow our desires all the time," said Haselton, giving dieting as a prominent example. "Humans are fully capable of doing that."
Men May Have a Gene for Monogamy
The notion that men have a gene to make them more monogamous created a stir when news of it came out toward the end of last year.
Having the gene RS3 334 may prompt a man to be more intimate with his partner and more likely to be married, according to a study of voles from the Karolinska Institute in Stockholm published last September.
That gene affects a hormone known as arginine vasopressin, which was found to make voles more likely to be interested in choosing a mate and parenting.
The study showed that the gene exists in humans, and men with the gene are more likely to be married, less likely to have conflicts in their marriage and more likely to have their partners report being satisfied with the relationship.
But should the monogamy gene even be a consideration in deciding if a man is right for a relationship?
"There are, of course, many environmental factors that will be more important than this gene variant when it comes to behaviors concerning pair-bonding," said Hasse Walum, the lead researcher on the study.
The gene might give science insight into relationships in the future, but it isn't likely to help women choose a man in the here and now.
"Our findings will not be very useful then trying to determine how happy men will be in future relationships, but we believe that our results could in the future help us to better understand the biology behind human-bonded relationships," said Walum.
How Much Does the Nose Know?
Is it important to find someone with different immunity genes?
Liking your significant other's scent is judged to be important by many, and some researchers have tied it to the likelihood of a couple staying together and the likelihood of someone cheating on his or her partner.
The reason, researchers have said, is that scents let us know about our partner's immune genes, and we desire someone with different genes in order to give our offspring the best possible immune system.
Some studies have even indicated -- by having women smell men's shirts -- that the nose plays a strong role in picking a mate.
But that research may be shortsighted.
A study published by researchers in the United Kingdom, France and China showed that Americans and Europeans may prefer the scents of people with a different immune system, but Africans they studied actually seemed to prefer mates who had similar immune systems.
So scent may play a role in mate selection, but it isn't clear how big a role or how its effects work.
So don't panic if you aren't enamored of the scent of your mate's dirty laundry.
Does a Big Chin Make for a Big Cheater?
Researchers at four North American universities published findings last month showing that women with more masculine facial features may be more promiscuous -- even when in a relationship -- but also found that men were typically able to spot this and judged these women to be less suitable as long-term partners.
Lorne Campbell, a psychology researcher at the University of Western Ontario and one of the authors of the research, noted that men were generally able to determine whether women were less sexually restrained -- confirmed by questionnaires given to the women -- despite only seeing pictures of their faces.
"In some cases, our desirability as a long-term mate can be conveyed through our facial features, to a certain degree," said Campbell.
One possible culprit is testosterone, a hormone typically found at higher levels in males, which has taken a beating recently as being responsible for everything ranging from wars to the current financial crisis.
The hormone, linked to more masculine faces (with features like more prominent chins) in women, is also implicated in making these same women more promiscuous, although Campbell noted that in his study they did not measure testosterone levels in the women who participated.
He said, however, that men dating a woman with a larger chin should not be concerned, and shouldn't use that fact in deciding if she is a good long-term partner.
"We're looking at averages across large samples, so it doesn't really apply to small samples," said Campbell. "It's difficult to use that to help you make good mating decisions."
Women and Men Are More Offended by Different Types of Infidelity
When Cynthia Rodriguez filed for divorce from all-star shortstop Alex in July, her divorce papers stated that "Alex has emotionally abandoned his wife and children."
Alex had been spending time with pop star Madonna, learning about Jewish mysticism, or kabala, and the bond the two stars formed apparently excluded his wife. Even if there was nothing sexual going on, it was enough, it seems, to push Cynthia to file divorce papers.
And A-Rod's alleged emotional infidelity may indeed have been worse than if he had a sexual affair.
"Men are particularly averse to sexual infidelity in relationships," said Campbell, explaining why, in his own research, men preferred women they perceived as less likely to be promiscuous. "Some research has shown that women are particularly averse to emotional infidelity."
The evolutionary reasons for this go back to the goals of men and women. Men, concerned about their genes being passed on, would like to avoid spending time raising children who are not their own.
Women, meanwhile, are concerned that when they have children, someone is there to help raise them, and so they want to avoid having their partner devote resources outside of the relationship.
Despite this, Campbell said, sexual infidelity by men in a relationship still tends to offend women, even if not as much as men.
Need some advice for the Valentine's weekend? We'll hook you up at the ABCNews.com OnCall+ Wellness Center.