The nervous system is not the only communication system within the body. A second is the endocrine system, which conveys signals from one part of the body to another using biochemicals. The biochemical signals affect activity in the body’s organs.

As you’ll see, the body’s two communication systems are linked; brain activity affects endocrine activity. This link is an important connection between mind and body.

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The biochemicals that the endocrine system uses for communication are hormones, which travel through the bloodstream and act as “messengers.” They carry messages to the body’s organs.

Hormone-based communication in the endocrine system differs from communication in the nervous system. The nervous system is fast and specific, whereas the endocrine system is slower and less specific. For example, suppose you’re on a playing field and see, out of the corner of your eye, a football headed straight for your head. Two things happen:

The different speeds of these bodily reactions reflect the two systems’ different communications mechanisms. The nervous system, as you learned, communicates electrically: Action potentials zip along axons. The endocrine system communicates chemically: Hormones float through the bloodstream. Electrical “zipping” is faster than chemical “floating.”

Why would your body need the slow endocrine system if it’s got the fast nervous system? Sometimes the brain needs to send signals that are widespread and long-lasting. If that flying football in our example were an opening kickoff and you were one of the players, you would want to arouse multiple biological systems that prepared you for prolonged physical activity. Because hormones spread throughout the body, they are well equipped for the job of increasing the supply of energy to multiple muscles.

Hormones are produced by glands, which are bodily organs that produce and secrete chemicals. The glands of the endocrine system are located throughout the body (Figure 3.26). Two are in the brain. The pineal gland (so named because its shape resembles that of a pine cone) produces a hormone called melatonin that influences patterns of sleeping and wakefulness. The amount of melatonin released is affected not only by signals from the brain, but also by environmental factors. Darkness increases melatonin release that, in turn, induces drowsiness. If people are exposed to bright lights, their melatonin levels decrease (Lewy et al., 1980).

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The brain also houses the pituitary gland, an endocrine gland that is tiny (about the size of a pea) but powerful. In fact, it is so powerful that it is commonly called the “master gland” of the endocrine system. Its power derives from the fact that the pituitary gland releases hormones that influence biological activity in other glands. These include glands that respond to stress, contribute to reproduction, and regulate the body’s use of energy. In addition, the pituitary is the point of contact between the nervous system and the endocrine system. Specifically, the brain’s hypothalamus releases chemical substances that affect pituitary gland activity. Through this chain of influence—from hypothalamus to “master gland” pituitary to additional glands—the brain can influence endocrine activity throughout the body.

The body’s other glands are located below the head, as shown in Figure 3.26:

Hormones clearly are central to biological functioning. What, though, is their psychological relevance? Why discuss them in a psychology textbook?

Hormones impact psychological functioning; they influence people’s moods, motives, and mental abilities. Particularly clear evidence of this comes from research on women’s hormone levels and psychological functioning. Levels of ovarian hormones vary across the menstrual cycle. If psychological functioning similarly varies across the menstrual cycle, in tandem with hormone levels, this would provide evidence that hormones influence psychology. Let’s look at two examples.

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A research team (Maki, Rich, & Rosenbaum, 2002) knew that estrogen can influence the growth of brain cells in areas of the brain needed for memory. They thus predicted that estrogen levels and memory performance would be linked. To test their prediction, they studied women at two time points: early in the menstrual cycle, when estrogen levels are low, and later in the cycle, when levels are high. At both times, women read a list of words and later performed a task that measured whether they retained information from the word list in memory. Women’s memory was found to vary across the menstrual cycle. As predicted, when estrogen levels were high, memory was superior (Maki et al., 2002).

The second example involves estrogen levels and styles of dressing and personal care. Researchers reasoned that, over the course of evolution, it was adaptive for women to appear attractive to mates during a particular time of the month: when they were most biologically fertile. During low-fertility periods, by contrast, it was adaptive to devote time to activities other than mating. The researchers hypothesized that, because we inherit psychological traces of the ancestral past, women today would still tend to pursue mates during biologically fertile periods, with this pursuit revealed in their styles of clothing and personal care. To test this hypothesis, the researchers took photos of women on both low- and high-fertility days in the given woman’s menstrual cycle. Afterward, raters were asked to judge, for each pair of photos (i.e., photos from the high- and low-fertility period for each woman), “In which photo is the person trying to look more attractive?” (Haselton et al., 2007, p. 42). The judges, of course, did not know which photo was taken during the high-fertility period.

In which photo were women trying to look more attractive? As predicted, it was the photo from the high-fertility period; to a significant degree, judges rated this photo as the one in which women were choosing fashion items to enhance their attractiveness (Haselton et al., 2007). Ratings indicated that, during high-fertility periods, women wore more fashionable clothes that showed more skin (Table 3.2).

As with studies of the nervous system, then, research on the endocrine system reveals interconnections between biological mechanisms that evolved in the past and psychological experiences in the contemporary social world.

Research shows that women dress differently during high- and low-fertility times in their menstrual cycles. Women more often wear fashionable and revealing clothing during high-fertility times of the month.

Source: Reprinted from Hormones and Behavior, 51:40–45, Haselton et al., Ovulatory shifts in human female ornamentation: Near ovulation, women dress to impress, © 2007, with permission from Elsevier

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