Most popular notions about the male brain are based on studies of men ages 18 to 22 -- undergrads subjecting themselves to experiments for beer money or course credit. But a man's brain varies tremendously over his life span, quickly contradicting the image of the single-minded sex addict that circulates in mainstream consciousness. From his wandering eye to his desire to mate for life, here's what you need to know about guys' minds. 1. More emotional While females are usually considered the more emotional gender, infant boys are more emotionally reactive and expressive than infant girls, researchers have found. Adult men have slightly stronger emotional reactions, too -- but only before they are aware of their feelings, found a 2008 study published in the Scandinavian Journal of Psychology that closely monitored facial expressions. Once the emotion reaches consciousness, however, men adopt a poker face. When young, boys likely learn to hide emotions that culture considers "unmanly." But tamping down emotion also spurs the body's "fight or flight" response. A man's strong reaction and subsequent suppression may ready him to handle a threat, theorize the 2008 study researchers at Lund University in Sweden. 2. More vulnerable to loneliness While loneliness can take a toll on everyone's health and brain, older men seem particularly vulnerable, said Dr. Louann Brizendine, a professor of clinical psychology at the University of California, San Francisco, and author of "The Male Brain" (Broadway, March 2010). Men tend to reach out less than women, which exacerbates loneliness and the toll it takes on their brains' social circuits, she said. Living with women may be particularly helpful. Men in stable relationships tend to be healthier, live longer and have hormone levels that may indicate decreased anxiety, studies have shown. Women might also be good for a guy's gonads. Male mice living with females remained fertile longer than their isolated cousins, found a study published in the Biology of Reproduction in 2009. 3. Focused on solutions While many studies suggest that women are more empathetic than men, Dr. Brizendine stresses this is not entirely true. The empathy system of the male brain does respond when someone is stressed or expressing a problem. But the "fix-it" region quickly takes over. "This hub does a Google search of the entire brain to come up with a solution," said Brizendine. As a result, men tend to be more concerned with fixing a problem than showing solidarity in feeling, she said. 4. Hard-wired to check out women While often linked to aggression and hostility, testosterone is also the hormone of the libido. And guys have six times the amount surging through their veins as women, said Pranjal Mehta, a social psychologist at Columbia University in New York. Mehta and colleagues found that testosterone impairs the impulse-control region of the brain. While it has yet to be studied, this may explain why, as Brizendine says, men ogle women as if on "auto-pilot." They often forget about the woman once she is out of their visual field, Brizendine said. 5. Must defend turf "Part of the male job, evolutionarily-speaking, is to defend turf," Brizendine said. More research is needed in humans but in other male mammals, the "defend my turf" brain area is larger than their female counterparts,' she said. While women too have fits of possessiveness, men are much more likely to become violent when faced with a threat to their love life or territory, she said. 6.Who's boss? An unstable hierarchy can cause men considerable anxiety, Brizendine said. But an established chain of command, such as that practiced by the military and many work places, reduces testosterone and curbs male aggression, she said. Pre-occupation with establishing pecking order, which starts as early as age 6, motivates the "male dance, where they are always putting each other down," Brizendine added. "It is better to be aggressive in a verbal jab than to duke it out," she said. 7. The mature male brain Over the course of evolution, men have needed to compete for status and mates while young and emphasize bonding and cooperation when mature, Mehta said. Men seem to agree; and psychological studies have shown that one-upmanship holds less appeal for older men. Instead, they pay more attention to relationships and bettering the community, Brizendine said. The change is likely aided by the slow natural decline in testosterone as a man ages. Mehta and colleagues found that men with high testosterone levels tend to be better at one-on-one competition, while those with lower levels excel at competitions requiring team cooperation. The study was published in the journal Hormones and Behaviour in 2009. 8. The father-to-be The male brain becomes especially primed for cooperation in the months before becoming a father. Fathers-to-be go through hormone changes -- prolactin goes up, testosterone goes down -- which likely encourage paternal behaviour, found a 2000 study in Evolution and Human Behaviour. The pheromones of a pregnant woman may waft over to her mate to spur these changes, said Brizendine, who was not involved with the study. The expecting mom might be repaying a favour: Even before she is pregnant, male pheromones cause good-mom neurons to sprout in the female brain, found a 2008 study published in the journal Hormones and Behaviour. 9. Daddy-play Daddy-specific ways of playing with their kids -- more rough-housing, more spontaneity, more teasing -- can help kids learn better, be more confidant, and prepare them for the real world, studies have shown. Also, involved dads lessen risky kids' sexual behaviour. Fathers that actively parent tend to have lower testosterone levels, report several cross-cultural studies. While it is not known if the hormone levels cause the behaviour or vice versa, researchers theorize that evolution has favoured involved dads. Human children are among the neediest of the animal kingdom and good dads optimize the chance that their offspring -- and their genes -- survive. 10. Covet wedding bells, too Women want to settle down, and men want to sow their wild oats forever, the refrain usually goes. But this might be one of the largest misconceptions stemming from the U.S. tendency of using undergrads as test subjects. Infidelities are most likely to occur before men hit 30, found a study of Bolivian men published in the Proceedings of the Royal Society in 2007. After that, men primarily focus on providing for their families, the study found. Of course, some men have a harder time with commitment than others -- a problem which could be genetic, according to a 2008 study in the Proceedings of the National Academy of Science. Men without the "promiscuity gene," an estimated 60 percent of the population, are more likely to marry. But that's not all. Both they and their wives are also more likely to report relative marital bliss, the researchers found. Unfortunately, the association is so small, said the study's lead researcher Hasse Walum of the Karolinska Institute in Sweden, "you can't use it for screening potential mates."