Several studies have indicated that males and females do act differently. One study even reported that men and women college students hold their books in different manners (1). Robert Trivers, an influential evolutionary biologist during the 1970's, suggested that males and females have evolved different patterns of behavior due to various levels of investment in reproduction and child-rearing (2) . According to Trivers' theory, female mammals are more invested in the reproductive process than male mammals because females must contribute a relatively large egg, milk, and blood to create offspring. Males, however, donate a relatively small amount of seamen. Since females must sacrifice more to reproduce, they are more likely to favor producing a smaller quantity of high quality offspring. In contrast, males have relatively little invested in each offspring, so producing a large quantity of offspring is favorable to males. Thus, theoretically, females are more selective of mates than males, and males must compete to win mates (2) . Because of this competition, males are thought to be more aggressive than females. Observation of human children has confirmed that young males tend to be more aggressive than young females (3) . In accordance with this pattern, preschool-aged boys are generally more willing to explore than preschool-aged girls, while girls tend to return to their mothers from exploration more frequently than boys (4) . For example, if a room was divided by some sort of barrier, with the mothers on one side and the children on the other, the boy children are more likely to try to find a way around the barrier, and the girl children are more likely to stand in the middle of the barrier and cry (4) . However, the behavior of preschool-aged children is not necessarily indicative of any innate, biological difference between males and females. The fact that the previously mentioned experiment involved mothers shows that these children have already been exposed to gender roles and expectations. Their behavior may reflect what they have been taught more than how they are biologically predispositioned to behave. Similar studies have found that little girls often take more time to say good-bye to their mothers before school, that boys are usually more active when they play, and that boys generally prefer to play with building blocks and vehicles (4) . While these different tendencies may reflect variations in neural organization, environmental influences, particularly social influences, may have had a role in shaping the development of the children.
Infants, who have had minimal exposure to social conventions, do display variation between male and female behavior. Human female infants tend to be more sensitive to touch than male infants are. One study reported that the least sensitive female infant in a sample may be more sensitive than the most sensitive male infant (4) . Female infants are often more sensitive to sound and are more easily agitated by noise than male infants (4) . Baby girls seem to be more social, as well. They are more inclined to "gurgle" at people and to recognize familiar faces than baby boys (4) . The behavioral differences between infants of different sexes appear very early in life, indicating that the mechanism controlling these behavioral patterns is innate and not learned from society. These studies show that males and females tend to behave differently from an early age, but they do not explain why their behavior differs.
Hormonal dissimilarity may be responsible for contrasts between stereotypical male and female behavior. Certain hormonal abnormalities, and the effects on behavior, indicate that hormones may, indeed, play a significant role in modifying behavior. A study of females with Adrenogenital Syndrome (AGS) illustrates the power of hormones. AGS results from prenatal over-production of androgens, causing female genitalia to be partially masculinized (3) . While the external deformities are generally corrected at birth, early exposure to androgens can result in life-long behavioral differences. According to one study, females with AGS tend to display stereotypical male behavioral traits, such as aggression and high energy levels (3) . Even though androgen exposure is a probable cause of the reported behavioral discrepancy, measures of aggression and energy are highly subjective and possibly influenced by a researcher's bias.
Hormones have two kinds of effects: organizational and activational (3) . Organizational effects modify the organism early in life, primarily influencing its anatomy. Activational effects are apparent later in life and influence the way previously established structures function (3) . Organizational and activational effects are not clearly delineated, because their effects can overlap. For example, female brains are "permanently transient" because of hormonal fluctuations that continually change the physiology of the brain (5) .
Hormones may not affect the brain directly. By altering other tissues or organs, such as the uterus, sensory input can be altered, affecting the observed behavior of the animal (6) . When hormones do act directly on the brain, they are target-specific, affecting only a particular part of the nervous system (6) . Hormones are often steroids that bind to particular areas and mediate genome expression, thereby modifying neural development (5) . Steroids can affect neuronal myelination, migrational patterns, growth, proliferation, and death of cells, and dendritic growth (5) . A variation in types of steroids and relative concentrations creates many complex differences between the sexes.
Androgens, produced by the testes in males and the adrenal cortex in both sexes, appear to have a profound organizational effect on early neural development (5) . Female rats exposed neonatally to androgens will mount other rats, which is a typical male mating behavior. Likewise, male rats deprived of androgen from an early age will exhibit lordosis, which is a typical female mating behavior (3). Males can be deprived of androgens by castration or prenatal exposure to alcohol or stress, which lower prenatal surges of testosterone in male fetuses (5) . Castrated males are much less likely to behave sexually because of a lack of testosterone, a principle androgen. Administration of testosterone will increase the level of sexual activity exhibited by a castrated male (6). Castrated males also display an increase in lordosis (5) .
Testosterone can be converted to estradiol, an estrogen, by aromatase, an enzyme (5) . The conversion of testosterone is essential to the masculinization of the male rat brain, and possibly the brains of other mammals (5) . Male rats deprived of aromatase develop female-like patterns of learning (5) . Alpha-fetoprotein (AFP), a blood protein, protects female rat pups from the masculinizing effects of estrogen (5) . AFP binds to estrogen and ensures that estrogen will not be absorbed by neurons as easily (5) . Synthetic estrogen, which does not bind to AFP, and high doses of estradiol can cause females to exhibit masculine mating behaviors, indicating that biosynthesized estradiol is essential in the process of neural masculinization (5) . Placement of estradiol in the hippocampus or cortex of neonatally castrated males reinstates a learning behavior typical of males that is generally absent in castrated males, further indicating that estradiol is an important chemical in the masculinization process (5) . The sexually dimorphic nucleus of the preoptic area (SDN-POA) is eight times larger in male rats than female rats. A female rat treated with testosterone or synthetic estrogen will have a larger SDN-POA (5) . Estrogen appears to enlarge the SDN-POA by preventing cell death in the area (5) .
Ovarian hormones are being increasingly recognized for their impact on neural development. Traditionally, testosterone and other androgens have been considered the primary chemicals responsible for neural sex differentiation (5) . Female development was believed to be "neutral" or "default", occurring because androgens were absent (5) . However, several studies suggest that ovarian hormones are essential to the feminization of the brain (5) . Proceptive behavior, including darting and ear wiggling, is typical of female rats (6) . If the ovaries of a female rat are removed, she will not engage in proceptive behavior or lordosis. Castrated males will perform lordosis, but will not engage in proceptive behavior, indicating that proceptive behavior is mediated by ovarian hormones (5) . Ovariectomized female rats will not act as typical females in open field tests or plus maze tests, further affirming the significance of ovarian hormones in feminization (5) . Before puberty, male and female rats have similar spatial learning abilities. After puberty, male spatial learning abilities do not change. In contrast, female spatial learning abilities decline, possibly due to a neural reorganization that occurs during this time because of ovarian hormonal changes (5) . When the ovaries of a female rat are removed early in life, the cortex of the rat is thicker than the cortex of an intact female rat (5) . Ovariectomized female rats that received estrogen supplements developed thin cortexes, while ovariectomized female rats that received progesterone supplements developed thick cortexes, indicating that estrogen limits the size of the cortex, but progesterone does not (5) . Males tend to have an asymmetrical cortex, with the right hemisphere larger than the left hemisphere. Females do not exhibit this pattern, unless deprived of ovarian hormones through ovarian removal (5) . These experiments suggest that ovarian hormones are as essential for feminization as androgens are for masculinization.
Curiously, estrogen causes masculinization in males and feminization in females. This is possible because of several differences between males and females. First, the level of estrogen is much higher in males because high levels of testosterone can be aromatized, compared with the relatively low level of estrogen in prepubescent rats (5) . Second, males and females have critical periods of estrogen sensitivity. Male rats are sensitive to estrogen relatively early in development, while female rats are sensitive later and for a longer period of time (5) . Finally, male and female rats have different receptor populations that are influenced by age (5) . These factors ensure different patterns of growth and development in males and females.
Behavioral differences between the sexes indicate that males and females differ in the structure and function of their nervous systems. Research supports this postulation. Yet, researching biological differences between the sexes is discouraged by some people who fear that women will be found biologically inferior, and that dangerous or hurtful behaviors such as "aggression, rape, and philandering" will be found to be innate, and therefore incurable (2) . However, limiting our understanding of the male and female nervous systems is not helpful. Understanding the nervous system aids in understanding behavior. Biology may explain behavior, but it does not justify it. If the origin of a harmful behavior is understood, a method of preventing the behavior can be found. The neurological differences between the sexes are considerably variable, including differences in patterns of mylination, and hippocampal and cortical morphology (7) . Men and women may be different neurologically, but their fundamental humanity is the same. The wide variability in differences ultimately creates a more interesting world.
2) Sex on the Brain: The Biological Differences between men and women
3) University of Plymouth, Department of Psychology: Biological Bases of Behavior, Psychosexual Differentiation
4) Excerpts from Brain Sex: The real difference between men and women
5) A role for ovarian hormones in sexual differentiation of the brain
6) University of Plymouth, Department of Psychology: Hormones & Sexual Behavior
7) Neural development and the influence of sex, hormones, and the environment
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