Psychology

The most developed theory to explain sex differences in humans, evolutionary psychology explains these differences as reflections of the demands of different social and physical environments on males and females in primeval times.  Evolutionary psychologists believe that males and females faced different demands from their environments, their respective reproductive statuses being the key elements of their lives (Denisiuk, 2004).  Thus, sex-specific mechanisms were evolved for humans to live with.  In fact, this is the reason for different behavioral patterns of the two sexes (Denisiuk).  Males and females developed differing strategies so as to ensure their success in reproduction and survival.  This is the reason for psychological differences between males and females.  After all, there are differences in social roles played by men and women (Denisiuk).
   
Thus, evolutionary psychologists claim that the differences between men and women are based on differing parental investment (Denisiuk).  The investment of women in reproduction outweighs the investment of men, hence females develop traits to help them improve their chances to reproduce offsprings that would survive (Denisiuk).  Males, on the contrary, happen to be less concerned with reproduction (Denisiuk).  Consequently, men are less choosy about mates (Denisiuk).  According to Denisiuk, Evolutionary psychologists view sex-evolved dispositions as psychological tendencies that have been built in genetically.  Environmental factors act as cues that interact with evolved predispositions to yield sex-typed responses This explains the difference in each sexs perspective on reproduction.    
   
Of course, evolutionary psychologists cannot scientifically prove that psychological differences between men and women originated in differing parental investment.  This is an assumption based on key concepts of the theory of evolution, that is, natural selection and survival of the fittest.

Evolutionary psychologists maintain that sex differences are based on differences in parental investment with the assumption that each human being is born to reproduce if, in fact, he or she is fit to survive.  This should explain sex differences in childhood as well as adulthood.  After all, male and female children, too, live with the ultimate goal to reproduce.  This goal is programmed or genetically built in.  Moreover, as reproduction requires parental investment, the theory of the origin of sex differences in parental investment claims that males and females develop psychological traits peculiar to their respective genders.  In other words, each male and female child is expected to develop traits peculiar to his or her gender in adulthood, so as to reproduce.
   
But, evolutionary psychology is not based on such assumptions alone.  In fact, evolutionary psychologists have conducted plenty of scientific studies on sex differences in parental investment.  Scientific research has shown, for example, that there are clear relationships between sexual strategies employed by individuals and their differing levels of parental investment (Geher, Fairweather, Mollette, Ugonabo, Murphy,  Wood, 2007, 19).  Denisiuk explains the relationship between mate selection and parental investment thus    
     
Evolutionary psychology indicates that characteristics that people seek in mates depend
on their sex and whether it is a short-term or a long-term mating. Women are limited in the
number of children they can have during their lifetime. Men have no restriction when it
comes to reproduction. Both men and women compete for their choice of mate. Women
will seek a mate who has resources to support their parental efforts, whereas men will seek
a mate for reasons different from wanting to be a parent. This establishes a difference in
views toward mating for each gender because each will have their own expectations A
 man has two possibilities for multiplying copies of his genes He can either aim for
quantity or quality of offsprings. With quantity, a man can impregnate as many women as
possible without staying around to help raise any of the children. With quality, a man can
stay with one female partner and have fewer children, but he will be present during their
upbringing Women have a limit on how many children they can have and a time limit on
when they can bear children. Because of these limits, women are strongly motivated to
ensure that the children they have will have the physical and psychological traits necessary
to survive and to be able to reproduce successfully. (Denisiuk)

Denisiuk sounds perfectly rational as this information on mate selection and parental investment she has provided must necessarily be part and parcel of the human belief system based on natural demands of reproduction.  Wang (2007) notes that the human mind is a collection of specialized mechanisms or computational algorithms, according to evolutionary psychologists (406).  The algorithms and mechanisms that the human mind comprises serve as tools to solve particular problems recurrent in hominid evolution (Wang, 406).  Like animals, human beings seek to satisfy their physical needs from birth until death.  Hence, some of the problems of survival that the human mind is programmed to resolve must include finding food, water, protection and mates.
   
The fact that the human mind is programmed to solve certain types of problems  according to evolutionary psychologists  does not mean that the human being is similar to a robot, designed to carry out some functions with built-in commands and instructions.  Rather, the theory of evolution refers to Homo sapiens or the wise man, gifted with the faculty of reason (Wang, 407).  Thus, females as rational beings are not only more emotionally expressive than males but also able to inhibit their emotional expression better than males.  This helps them to control their sexual desires before males who are not their sexual partners or spouses.  It also helps them to inhibit their aggression before their infants, thereby giving them an evolutionary advantage (Leigh, 2003).  Psychological stress is detrimental to brain development in childhood hence women who control their negative emotions before their children are better able to survive through their offspring (Barber, 2005, 144).
   
As a matter of fact, both men and women are rational when it comes to the question of survival through reproduction.  Geher et. al. consider whether males and females are always conscious of their decisions regarding reproduction, seeing that psychological sex differences in parental investment are said to be innate (18-29).  Although the question of conscious versus unconscious attitudes toward reproduction has not been completely resolved, the fact remains that there are tendencies peculiar to each sex.  Thus, Denisiuk writes that women tend to prefer males with symmetrical facial features, for example, unblemished and clear skins, as such features are indicative of good health.  Healthy people have good genes.  But, women also prefer males with masculine features, for example, facial hair and muscular bodies.  Such features are indicative of adequate testosterone required for fertility.  Moreover, women prefer older men who are intelligent and have high social standing.  They show preference for wealthy men, too, for the reason that they would like their mates to have sufficient power and resources to take care of their offsprings (Denisiuk).
   
Women search for mates to spend their entire lives with.  Males, on the other hand, are chiefly interested in impregnating mates to have offsprings.  They seek young women for this reason.  Females with full breasts and hips in addition to small waists are preferred as these features are indicative of adequate estrogen for successful reproduction.  Men also pay attention to facial features of women, showing preference for females with clear skins, etc. (Denisiuk).
   
Also according to evolutionary psychologists, males and females have different reactions to sexual infidelity (Denisiuk).  If a woman strays, her mate would experience sexual jealousy for the reason of uncertainty in paternity of their children (Denisiuk).  If a man strays, his mate does not experience such uncertainty.  The child belongs to the mother.  However, women experience sexual jealousy if their mates are interested in other females for the reason that their mates commitments to their children would be reduced if this happens (Denisiuk).  Men would expend less of their time, attention, energy and resources on their children if they are interested in other women, who too may reproduce with them (Denisiuk).  As men are traditionally the breadwinners of their families, women believe that their childrens survival is at stake if their mates are interested in other females.  Thus, women are expected to react with negative emotions whenever they or their offsprings are deprived of necessary support from their mates, including emotional support.  Males, on the contrary, are expected to show anger if their wives are believed to straying (Denisiuk).
   
Another difference between males and females is, of course, the fact that fertility of females declines with age but remains unchanged for males (Walsh, 2009).  Hence, Turner  McAndrew (2006) explain that young women tend to be choosier when it comes to mate selection (198).  Older females, on the other hand, may opt to invest in children who may or may not survive (Turner  McAndrew, 198).  As evolutionary psychologists assume that decisions about parental investment are largely unconscious, older women that choose to invest in children who may or may not survive may feel guided by the emotional state of love (Turner  McAndrew, 198).  Even men may feel that they are guided by such emotional states as they make parental investment decisions (Turner  McAndrew, 198).  In other words, the unconscious need to reproduce may or may not enter the human mind through the process of parental investment decision-making, which includes mate selection.
   
Wang notes that parental investment decision-making is based on the concept of utility maximization studied by economists (408).  In this framework, the expected utility of a choice option is calculated as the sum of expected values of the options outcomes weighted by corresponding probabilities of the outcomes (Wang, 408).  As parental investment decisions depend primarily on women who must choose between several men who court them, it is essential for females to estimate discrepancy between their expectations and actual investments made by their partners in the future (Wang, 408).  A male produces enough sperm to inseminate thousands of women through his lifetime (Kenyon, 2006).  A female, on the contrary, cannot produce as many children (Kenyon).  Hence, women desire great investment from their mates.  In fact, female expectations in this regard may exceed average investment made by males.  It is for this reason that a woman should estimate discrepancy between her expectations and actual investment made by her mate in the future as she makes her parental investment decision in the present based on the concept of utility maximization (Wang, 408).
   
If a woman is unable to accurately estimate such discrepancy, the survival of her future offsprings may be at stake.  After all, the female needs her mate to share his resources with her children so as to protect and nourish them.  But, the male who tends to have a higher number of friends than the female is more likely to get interested in other females at the same time as he is attached to the mother of his children (Evolutionary Psychology and Infidelity).  Therefore, a woman who knows that her mate is likely to get interested in other females would not place a high value on him.  She would estimate high discrepancy for such a mate, according to the economic formula already explained.  The laws of supply and demand say that the more a specific commodity is available, the cheaper it becomes and less valuable to possess (Evolutionary Psychology and Infidelity).  As the principles of economics are based on the behavior of humans, they must be applicable in evolutionary psychology (Evolutionary Psychology and Infidelity).    
   
Thus, parental investment decisions are rational, regardless of what males and females are consciously feeling or thinking as they go through the decision-making process.  Given that the two sexes have different types of investments to make in their children, men and women tend to think differently, not only as they go through the parental investment decision-making process but also otherwise.  In fact, evolutionary psychologists claim that sex differences are based in parental investment decisions.  In other words, all humans are genetically programmed with the need to produce offsprings.  Although biological differences between males and females result in psychological differences, the two sexes share the need to survive through their children.