Until this point, we have discussed the brain as if it were some fixed and unchanging organ, the same in everyone and essentially the same organ at birth as at age 8, 18, 28, or 98. Indeed, because we are all human and most of us share common early experiences—learning to walk, talk, deal with school and family members—our brains are fundamentally similar in their basic organization. Yet, we also have differing experiences as a result of growing up rich or poor, male or female, nurtured or neglected, and these experiences take place within a particular culture that shapes our values, skills, and opportunities. Such differences can affect the brain's wiring and how it is used.

Our brains are not fully formed at birth. During infancy, synapses proliferate at a great rate (see Figure4.14). Neurons sprout new dendrites, creating new synapses and producing more complex connections among the brain's nerve cells (Bock et al., 2014; Greenough & Black, 1992; Kostović & Judaš, 2009; Rosenzweig, 1984). New learning and stimulating environments promote this increase in complexity. Then, during childhood, synaptic connections that are useful for helping the child respond to the environment survive and are strengthened whereas those that are not useful wither away, leaving behind a more efficient neural network. In this way, each brain is optimized for its environment. This plasticity, the brain's ability to change in response to new experiences, is most pronounced during infancy and early childhood, and has a resurgence in adolescence, but it continues throughout life.

The brain's plasticity may help explain why some people who cannot recall simple words after a stroke may be speaking almost normally within months, and why some who cannot move an arm after a head injury may regain full use of the limb after physical therapy. Their brains have apparently rewired themselves to adapt to the damage (Liepert et al., 2000). Plasticity is also apparent in some people who have been blind or deaf from birth or early childhood. In the period of rapid development after birth, connections form between the eyes and the visual cortex, and between the ears and the auditory cortex—but also between the eyes and the auditory cortex, and the ears and the visual cortex. (How this might affect the infant's perception, if it does, is a mystery.) Typically, experience strengthens connections between the eyes and the visual cortex, and between the ears and auditory cortex, and prunes away the other two types (Innocenti & Price, 2005). The intriguing question, therefore, is what happens in the visual cortex of blind people? Is it able to respond to sound because it is not receiving sight?

To answer this question, researchers used PET scans to examine the brains of people as they localized sounds heard through speakers (Gougoux et al., 2005). Some people were sighted but others had been blind from early in life. When they heard sounds through both ears, activity in the occipital cortex (an area associated with vision) decreased in the sighted people but not in the blind ones. When one ear was plugged, the blind people who did especially well at localizing sounds showed activation in two areas of the occipital cortex; neither sighted people nor blind people with ordinary ability showed that activation. What's more, the degree of activation in these regions was correlated with the blind people's accuracy on the task (see Figure4.15). The brains of those with the best performance had apparently adapted to blindness by recruiting visual areas to take part in activities involving hearing—a dramatic example of plasticity.

Figure 4.15

Adapting to Blindness

In sighted people, the visual areas in the brain are quiet during tasks requiring hearing or touch (such as touching Braille letters). But researchers wondered what would happen to those visual areas if the volunteers were blindfolded for 5 days. The answer was that by day 5, those visual areas had become active during the tasks, and after the blindfolds were removed, the visual centers once again quieted down (Pascual-Leone et al., 2005). The visual areas of the brain apparently possess the computational machinery necessary for processing nonvisual information, but this machinery remains dormant until circumstances require its activation (Amedi et al., 2005). When people have been blind for most of their lives, new connections may form, permitting lasting structural changes in the brain's wiring. The opposite is true of deaf people. When neuroscientists asked deaf and hearing people to study sets of moving dots, the brains of deaf people showed activity in the auditory cortex, but the brains of hearing people did not (Finney, Fine, & Dobkins, 2001).

This research teaches us that the brain is a dynamic organ: Its circuits are continually being modified in response to information, challenges, and changes in the environment. As scientists come to understand this process better, they may be able to apply their knowledge by designing improved rehabilitation programs for people with sensory impairments, developmental disabilities, and brain injuries.

Many best-selling books today claim that men and women have different brains. The Female Brain was so successful that it inspired a sequel, The Male Brain. It's a Baby Girl! claims that “Without testosterone interfering, your daughter develops not only female genitalia but a decidedly female brain . . . [one] that will direct her female approach to the world” (quoted in Fine, 2010). (Critical thinkers might wonder what a “female approach to the world” is, that is, one unaffected by a woman's religion, culture, social class, age, generation, occupation, nationality, or education.) Popular books about leadership, marital problems, parenting, and education likewise claim that males and females have hardwired brain differences that explain, among other things, women's allegedly superior intuition and empathy, women's love of talking about feelings and men's love of talking about sports, women's greater verbal ability, men's greater math ability, and why men won't ask for directions when they're lost. Some writers call upon brain science to argue that the sexes are so different that they should be segregated into different schools.

What is a layperson to make of these arguments, which are often presented with lots of fMRI images and other pictures of the brain? Of course there are innumerable average differences in men's and women's experiences and behavior. Unfortunately, as a panel of eminent neuroscientists and other psychological scientists discussed at a professional meeting, ideology often gets in the way of interpreting research on sex differences and the brain: Some people worry that the research can be used to justify sexism (a legitimate concern, given those oversimplified pop-psych books), and others argue, just as legitimately, that ignoring the evidence is antiscientific and an impediment to improving the lives and health of both women and men (Saletan, 2011).

To evaluate this issue intelligently, we need to ask two separate questions: Do the brains of males and females differ, on average, in structure or function? And if so, what, if anything, do those differences have to do with men's and women's behavior, abilities, ways of solving problems, or anything else that matters in real life?

The answer to the first question is yes. Many anatomical and biochemical sex differences have been found in animal and human brains, in both their structure and function (Cahill, 2012; Luders et al., 2004). Some appear to be universal: A study combining fMRI data from 24 laboratories and more than 1,000 people worldwide—in Australia, China, England, Finland, Germany, the United States, and Wales—showed that when their brains are at rest (i.e., not engaged in doing any specific mental work), men and women have different patterns of activity across the brain as a whole (Biswal et al., 2010). Using fMRI, scientists can infer that two regions are connected if they become active at the same time and also inactive at the same time. In this huge international study, people of all ages, both sexes, and from all cultures showed remarkably similar patterns of connectivity. But there were also significant differences between the patterns seen in men and women, such as in connections between the cortex and the amygdala.

In addition, parts of the frontal lobes are larger in women, relative to the overall size of their brains, whereas parts of the parietal cortex and the amygdala are larger in men (Goldstein et al., 2001; Gur et al., 2002; Kim et al., 2012). Women also have more cortical folds in the frontal and parietal lobes (Luders et al., 2004). On average, men have more neurons than women do in the cortex, and some researchers speculate that this difference contributes to the sex difference in spatial abilities, such as skill at mentally rotating objects (Burgaleta et al., 2012). Finally, the amygdala appears to be lateralized according to sex. In men, the right amygdala keeps getting input from the rest of the brain; in women, the left amygdala gets that input. This lateralization seems to predispose men and women to encode and remember emotional information differently. In men, better memory is associated with greater activity in the right-hemisphere amygdala, but in women, better memory is associated with the left-hemisphere amygdala (Cahill et al., 2004).

Is your own male or female brain spinning yet? The bottom line is that average sex differences in the brain do exist. But we are still left with our second question: Overall, what do the differences mean for the behavior or personality traits of men and women in ordinary life? When you hear or read popular accounts of “sex and the brain,” keep these cautions in mind:

1. Many supposed differences between men and women in intuition, abilities, and traits are stereotypes; they mislead because the overlap between the sexes is often greater than the difference between them. Even when gender differences are statistically significant, they are often quite small in practical terms. And national differences may be greater than sex differences. Thus, boys are somewhat better than girls in math in the United States, Taiwan, and Japan, but the bigger difference is between countries, with Taiwanese and Japanese girls outscoring American boys (Else-Quest, Hyde, & Linn, 2010). Some supposed differences, on closer inspection, even disappear. Are women more talkative than men, as many pop-psych books about the sexes assert? To test this assumption, psychologists wired up a sample of men and women with voice recorders that tracked their conversations while they went about their daily lives. The sexes did not differ in the number of words spoken; women and men alike used about 16,000 words per day on average, with large individual differences among the participants (Mehl et al., 2007).

2. A brain difference does not necessarily produce a difference in behavior or performance. In many studies, males and females have shown different patterns of brain activity while they are doing something or while an ability is being tested, but they have not differed in the behavior or ability in question. The different patterns of activity are of great interest to scientists, who, after all, want to understand the mechanisms that produce behavior. If six routes lead to Rome, they want to know all six; laypeople just want to get there in time for lunch. The problem occurs when laypeople wrongly infer that a brain difference is the reason for a behavioral difference—one that doesn't exist! (Fine, 2010). Researchers used MRI to examine the brains of men and women who had equivalent IQ scores. They found some differences, such as that women's brains had more white-matter areas related to intelligence, whereas men's brains had more gray-matter areas related to intelligence (Haier et al., 2005). That is interesting, yet the sexes do not differ in overall intelligence. The researchers concluded that brains may be organized differently yet produce the same intellectual abilities.

3. Differences in the brain do not account for differences in behavior across situations. Consider the example of empathy, a skill central to the female stereotype. Most people will tell you that women in some generic way are better than men at empathy and intuition, and on self-report questionnaires, women are more likely than men to describe themselves as being high in empathy. Unfortunately, what people say about themselves, on any trait or behavior from kindness and altruism to obedience and cruelty, is typically unrelated to how they actually behave in various situations. When we hear that women are hardwired to be empathic, therefore, we need to ask, which women? Under what circumstances? Empathy toward whom? Women are not more empathic toward their enemies, familial or national, than men are. Over and over, if you watch what people do rather than what they say they would do, and vary the situations in which they do it, gender differences fade (Fine, 2010; Jordan-Young, 2010).

4. The eternal problem of cause and effect: Some male–female differences in the brain can be the result rather than the cause of behavioral differences. As we have seen, experiences and cultural influences are constantly sculpting the circuitry of the brain, affecting the way brains are organized and how they function. Women and men, of course, often have different experiences in childhood and throughout their lives. Thus, when researchers find a sex difference in brain structure or function, they cannot automatically assume that the difference is innate or unchangeable.

5. The elusive brain difference: Now you see it, now you don't. People have an understandable tendency to think that if a study of a dozen men's and women's brains finds an average difference, that result is generalizable to everyone. After all, a brain is a brain, isn't it? But research on brains, as on anything else, must be replicated, and sometimes the results are surprising. Researchers once were convinced that the corpus callosum of men and women differed in size, but early findings were not replicated. Researchers once thought the amygdala was the “anger center”; then they thought it was the “fear center”; now, as you read earlier in this chapter, they understand its more varied functions. Researchers once were convinced that men's brains were less lateralized than women's brains, especially in tasks involving language, meaning that women use both sides of the brain when they are doing a task and men only one side. But meta-analyses and large-scale studies have failed to confirm what “everyone knew” about lateralization (Chiarello et al., 2009; Sommer et al., 2004, 2008).

One reason that scientists differ in their interpretation of the research is that some are focusing on the differences between the sexes and others are focusing on the similarities. Both sides may be right. Yet both would agree that we all should avoid oversimplifying, jumping to conclusions, and thinking in either–or terms (either men and women have “different” brains or we are all exactly alike). Critical thinking skills are something both sexes can learn.

We think you'll agree that findings about the brain are pretty fascinating, and we will discuss many others in the rest of this book. However, these findings should not deflect attention from all the other influences that make us who we are, for better or worse: our relationships, our experiences, our standing in society, our culture. Keep in mind (as well as in your brain) that analyzing a human being in terms of physiology alone is like analyzing the Eiffel Tower solely in terms of the rivets that were used to build it. Even if we could monitor every cell and circuit of the brain, we would still need to understand the circumstances, thoughts, and cultural rules that affect whether we are gripped by hatred, consumed by grief, lifted by love, or transported by joy.