I. The Nervous System: On Being Wired

The nervous system consists of the brain, the spinal cord, and other parts that make it possible for us to receive information from the world outside and to act on the world. It is composed of cells, most of which are neurons.

A. Neurons: Into the Fabulous Forest

Neurons are specialized cells of the nervous system that conduct impulses. Neurons receive "messages" from a number of sources and they can pass these messages along in a complex biological dance. Most of them are found in the brain. The nervous system also contains glial cells. Glial cells remove dead neurons and waste products from the nervous system; nourish and insulate neurons; form a fatty, insulating substance called myelin; and play a role in neural transmission of messages.

Most neurons include a cell body, dendrites, and an axon. The cell body contains the core or nucleus of the cell. The nucleus uses oxygen and nutrients to generate the energy needed to carry out the work of the cell. Anywhere from a few to several hundred short fibers, or dendrites, extend like roots from the cell body to receive incoming messages from thousands of adjoining neurons. Each neuron has an axon that extends like a trunk from the cell body. Axons are very thin and end in small, bulb-shaped structures called axon terminals or terminal buttons. Neurons carry messages in one direction only: from the dendrites or cell body through the axon to the axon terminals. The messages are then transmitted from the terminal buttons to other neurons, muscles, or glands.

Myelin

Myelin is a fatty substance that encases and insulates axons, facilitating transmission of neural impulses. The myelin sheath minimizes leakage of the electrical current being carried along the axon, thereby allowing messages to be conducted more efficiently.

Afferent and Efferent Neurons

Afferent neurons are neurons that transmit messages from sensory receptors to the spinal cord and brain; also called sensory neurons. Efferent neurons are neurons that transmit messages from the brain or spinal cord to muscles and glands; also called motor neurons. Remember that they are the "SAME." That is, Sensory is to Afferent as Motor is to Efferent.

B. The Neural Impulse: "The Body Electric"

Neural impulses are messages that travel along neurons are electrochemical in nature. Neural impulses are messages that travel within neurons at somewhere between two and 225 miles an hour.

An Electrochemical Voyage

The process by which neural impulses travel is electrochemical. Neurons and body fluids contain ions—positively or negatively charged atoms. The difference in electrical charge readies, or polarizes, a neuron for firing by creating an internal negative charge in relation to the body fluid outside the cell membrane. The electrical potential across the neural membrane when it is not responding to other neurons—its resting potential—is about -70 millivolts in relation to the body fluid outside the cell membrane.

When an area on the surface of the resting neuron is adequately stimulated by other neurons, the cell membrane in the area changes its permeability to allow positively charged sodium ions to enter. Thus, the area of entry becomes positively charged, or depolarized, with respect to the outside. The electrical impulse that provides the basis for the conduction of a neural impulse along an axon of a neuron is termed its action potential.

Firing: How Messages Voyage From Neuron to Neuron

The conduction of the neural impulse along the length of a neuron is what is meant by firing. Neurons also fire, but instead of having a barrel, a neuron has an axon. Instead of discharging a bullet, it releases neurotransmitters. However, neurons will not fire unless the incoming messages combine to reach a certain strength, which is defined as the threshold at which a neuron will fire. Every time a neuron fires, it transmits an impulse of the same strength. This occurrence is known as the all-or-none principle. Neurons fire more often when they have been stimulated by larger numbers of other neurons. For a few thousandths of a second after firing, a neuron is in a refractory period; that is, it is insensitive to messages from other neurons and will not fire.

The Synapse: On Being Well-Connected

A neuron relays its message to another neuron across a junction called a synapse. A synapse consists of an axon terminal from the transmitting neuron, a dendrite, or the body of a receiving neuron, and a fluid filled gap between the two that is called the synaptic cleft.

C. Neurotransmitters: The Chemical Keys to Communication

Sacs called synaptic vesicles in the axon terminals contain neurotransmitters— the chemical keys to communication. Each kind of neurotransmitter has a unique chemical structure, and each can fit into a specifically tailored harbor, or receptor site, on the receiving cell. "Loose" neurotransmitters are usually either broken down or reabsorbed by the axon terminal (a process called reuptake). Some neurotransmitters act to excite other neurons—that is, to cause other neurons to fire. Other neurotransmitters inhibit receiving neurons. That is, they prevent the neurons from firing. Some neurotransmitters that are of interest to psychologists: acetylcholine (ACh), dopamine, norepinephrine, serotonin, GABA, and endorphins.

Acetylcholine (ACh) is a neurotransmitter that controls muscle contractions. It is excitatory at synapses between nerves and muscles that involve voluntary movement but inhibitory at the heart and some other locations. ACh is normally prevalent in a part of the brain called the hippocampus, a structure involved in the formation of memories. When the amount of ACh available to the brain decreases, as in Alzheimer's disease, memory formation is impaired.

Dopamine is a neurotransmitter that acts in the brain and affects the ability to perceive pleasure, voluntary movement, and learning and memory. Deficiencies of dopamine are linked to Parkinson's disease, in which people progressively lose control over their muscles. The psychological disease schizophrenia is characterized by confusion and false perceptions, and it has been linked to dopamine.

Norepinephrine is produced largely by neurons in the brain stem and acts both as a neurotransmitter and as a hormone. It is an excitatory neurotransmitter that speeds up the heartbeat and other body processes and is involved in general arousal, learning and memory, and eating. Excesses and deficiencies of norepinephrine have been linked to mood disorders. The stimulants cocaine and amphetamine ("speed") boost norepinephrine (as well as dopamine) production, increasing the firing of neurons and leading to persistent arousal.

Serotonin is a neurotransmitter that is involved in emotional arousal and sleep. Deficiencies of serotonin have been linked to eating disorders, alcoholism, depression, aggression, and insomnia.

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that may help calm anxiety reactions. Tranquilizers and alcohol may quell anxiety by binding with GABA receptors and amplifying its effects.

Endorphins inhibitory neurotransmitters. The word endorphin is the contraction of endogenous morphine. Endorphins occur naturally in the brain and in the bloodstream and are similar to the narcotic morphine in their functions and effects. They lock into receptor sites for chemicals that transmit pain messages to the brain. Endorphins may also increase people's sense of competence, enhance the functioning of the immune system, and be connected with the pleasurable "runner's high" reported by many long-distance runners.

Billions upon billions of axon terminals are pouring armadas of neurotransmitters into synaptic clefts at any given time. The combined activity of all these neurotransmitters determines which messages will be transmitted and which ones will not.

D. The Parts of the Nervous System

The nervous system consists of the brain, the spinal cord, and the nerves linking them to the sensory organs, muscles, and glands. The brain and the spinal cord make up the central nervous system. The sensory (afferent) neurons, which receive and transmit messages to the brain and spinal cord, and the motor (efferent) neurons, which transmit messages from the brain or spinal cord to the muscles and glands, make up the peripheral nervous system. In the comparison of the nervous system to a computer, the peripheral nervous system makes up the nervous system's peripheral devices.

The Peripheral Nervous System: The Body's Peripheral Devices

The peripheral nervous system consists of sensory and motor neurons that transmit messages to and from the central nervous system. The two main divisions of the peripheral nervous system are the somatic nervous system and the autonomic nervous system.

The somatic nervous system contains sensory (afferent) and motor (efferent) neurons. It transmits messages about sights, sounds, smells, temperature, body positions, and so on, to the central nervous system. Messages transmitted from the brain and spinal cord to the somatic nervous system control purposeful body movements.

The autonomic nervous system (ANS) also has afferent and efferent neurons and regulates the glands and the muscles of internal organs. Thus, the ANS controls activities such as heartbeat, respiration, digestion, dilation of the pupils. The ANS also has two branches, or divisions: sympathetic and parasympathetic. These branches have largely opposing effects.

The sympathetic division is most active during processes that involve spending body energy from stored reserves, such as a fight or flight response. The parasympathetic division is most active during processes that replenish reserves of energy, such as eating.

The Central Nervous System: The Body's Central Processing Unit

The central nervous system consists of the spinal cord and the brain. The spinal cord is a true "information superhighway"—a column of nerves as thick as a thumb. It transmits messages from sensory receptors to the brain and from the brain to muscles and glands throughout the body. A spinal reflex is an unlearned response to a stimulus that may require only two neurons—a sensory neuron and a motor neuron. In some reflexes, a third neuron, called an interneuron, transmits the neural impulse s from the sensory neuron through the spinal cord to the motor neuron. The spinal cord and brain contain gray matter and white matter. Gray matter consists of non-myelinated neurons. White matter is composed of bundles of longer, myelinated (and thus whitish) axons that carry messages to and from the brain.

II. The Brain: Wider Than the Sky

Accidents provide unplanned—and uncontrolled— opportunities of studying the brain. Still, scientists learn more about the brain through methods like experimentation, electroencephalography, and brain scans.

A. Experimenting with the Brain

The results of disease and accidents (as in the case of Phineas Gage) have shown that brain injuries can be connected with changes in behavior and mental processes. Because the brain has no receptors for pain, surgeon Wilder Penfield (1969) was able to stimulate parts of human brains with electrical probes. As a result, his patients reported perceiving certain memories.

The Electroencephalograph (EEG)

Penfield stimulated parts of the brain with an electrical current and asked people to report what they experienced. Researchers have used the electroencephalograph (EEG) to record the natural electrical activity of the brain. The EEG detects minute amounts of electrical activity—called brain waves—that pass between the electrodes.

Brain-Imaging Techniques

In the latter years of the 20th century, researchers developed imaging techniques that use the computer's ability to generate images of the parts of the brain from sources of radiation.

Computerized axial tomography (CAT or CT scan) passes X-rays through the head and measures the structures that reflect the beams from various angles, generating a three-dimensional image. The CAT scan reveals deformities in shape and structure that are connected with blood clots, tumors, and other health problems.

A second method, positron emission tomography (PET scan) forms a computer-generated image of the activity of parts of the brain by tracing the amount of glucose used (or metabolized) by these parts. To trace the metabolism of glucose, a harmless amount of radioactive compound, called a tracer, is mixed with glucose and injected into the bloodstream. When the glucose reaches the brain, the patterns of activity are revealed by measurement of the positrons—positively charged particles—that are given off by the tracer.

A third imaging technique is magnetic reasoning imaging (MRI) where the person lies in a powerful magnetic field and is exposed to radio waves that cause parts of the brain to emit signals, which are measured from multiple angles. MRI relies on subtle shifts in blood flow. Functional MRI (fMRI) provides a more rapid picture and therefore enables researchers to observe the brain "while it works" by taking repeated scans while subjects engage in activities such as mental processes and voluntary movements.

B. A Voyage through the Brain

Hindbrain is where the spinal cord rises to meet the brain. Here three major structures are found—the medulla, the pons, and the cerebellum. Many pathways pass through the medulla to connect the spinal cord to higher levels of the brain. The medulla regulates basic functions such as heart rate, blood pressure, and respiration. The pons is a bulge in the hindbrain that lies forward of the medulla. The pons transmits information about body movement and is involved in functions related to attention, sleep and arousal, and respiration. Behind the pons lies the cerebellum. The cerebellum has two hemispheres that are involved in maintaining balance and controlling motor (muscle) behavior.

As a tour the hindbrain is taken, the lower part of the reticular formation is also found. That is where the reticular formation begins, but it ascends through the mid-brain into the lower part of the forebrain. The reticular formation is vital in the functions of attention, sleep, and arousal.

Key areas of the forward most part of the brain, or forebrain, are the thalamus, the hypothalamus, the limbic system, and the cerebrum. The thalamus is located near the center of the brain and could be said to lie between the forebrain and the midbrain. The thalamus serves as a relay station for sensory stimulation. The hypothalamus lies beneath the thalamus and above the pituitary gland. It is vital in the regulation of body temperature, concentration of fluids, storage of nutrients, and motivation and emotion. Experimenters learn many of the functions of the hypothalamus by implanting electrodes in parts of it and observing the effects of electrical stimulation. They have also found that the hypothalamus is involved in hunger, thirst, sexual behavior, caring for offspring, and aggression.

Psychologists James Olds and Peter Milner were attempting to implant an electrode in a rat's reticular formation to see how stimulation of the area might affect learning. Olds missed his target and found a part of the animal's hypothalamus instead. Olds and Milner dubbed this area the "pleasure center" because the animal would repeat whatever it was doing when it was stimulated.

The limbic system forms a fringe along the inner edge of the cerebrum and is fully evolved only in mammals. It is made up of several structures, including the amygdala, hippocampus, and parts of the hypothalamus. It is involved in memory, emotion, and in the drives of hunger, sex, and aggression. The amygdala is near the bottom of the limbic system and looks like two little almonds. It is connected with aggressive behavior in monkeys, cats, and other animals. The amygdala is also connected with vigilance. It is involved in emotions, learning, and memory, and it behaves something like a spotlight, focusing attention on matters that are novel and important to know more about. The cerebrum is responsible for thinking and language. The surface of the cerebrum—the cerebral cortex—is wrinkled, or convoluted, with ridges and valleys. Valleys in the cortex are called fissures. A key fissure almost divides the cerebrum in half, creating two hemispheres. The hemispheres are connected by the corpus callosum, a bundle of some 200 million nerve fibers.

C. The Cerebral Cortex

The cerebral cortex is the outer coating of the cerebrum. The cerebral cortex is involved in almost every bodily activity, including most sensations and responses. The cerebral cortex has two hemispheres, left and right. Each of the hemispheres is divided into four lobes.

The frontal lobe lies in front of the central fissure and the parietal lobe behind it. The temporal lobe lies below the side, or lateral, fissure—across from the frontal and parietal lobes. The occipital lobe lies behind the temporal lobe and behind and below the parietal lobe. Just behind the central fissure in the parietal lobe lies a sensory area called the somatosensory cortex, which receives messages from skin sensors all over the body. The left hemisphere controls, acts on, and receives inputs from the right side of the body. The right hemisphere controls, acts on, and receives inputs from the left side of the body. The motor area of the cerebral cortex, or motor cortex, lies in the frontal lobe, just across the valley of the central fissure from the somato-sensory cortex.

Thinking, Language, and the Cortex

Areas of the cerebral cortex that are not primarily involved in sensation or motor activity are called association areas. They make possible the breadth and depth of human learning, thought, memory, and language. The association areas in the prefrontal region of the brain—that is, in the frontal lobes are the brain's executive center. It appears to be where people solve problems and make plans and decisions.

Language Functions

Two key language areas lie within the hemisphere of the cortex that contains language functions (usually the left hemisphere): Broca's area and Wernicke's area. Damage to either area is likely to cause an aphasia—that is, a disruption in the ability to understand or produce language.

Wernicke's area lies in the temporal lobe near the auditory cortex. It responds mainly to auditory information (sounds). Broca's area is located in the frontal lobe, near the section of the motor cortex that controls the muscles of the tongue, throat, and other areas of the face used when speaking. Broca's area processes the information and relays it to the motor cortex. People with damage to Wernicke's area may show Wernicke's aphasia, which impairs their abilities to comprehend speech and to think of the proper words to express their own thoughts. When Broca's area is damaged, people usually understand language well enough but speak slowly and laboriously, in simple sentences. This pattern is termed Broca's aphasia. A part of the brain called the angular gyrus lies between the visual cortex and Wernicke's area. The angular gyrus "translates" visual information, as in perceiving written words, into auditory information (sounds) and sends it on to Wernicke's area.

D. Left Brain, Right Brain?

The notion is that the hemispheres of the brain are involved in very different kinds of intellectual and emotional functions and responses. According to this view, left-brained people would be primarily logical and intellectual. Right brained people would be intuitive, creative, and emotional. Those people who are fortunate enough to have their brains "in balance" would presumably have the best of it—the capacity for logic combined with emotional richness. Like many other popular ideas, the left-brain versus right-brain notion is exaggerated. The functions of the left and right hemispheres overlap to some degree, and they tend to respond simultaneously.

E. Handedness

Being left-handed was once seen as a deficiency. Being left-handed appears to provide a somewhat-greater-than-average probability of language problems, such as dyslexia and stuttering, and health problems such as migraine headaches and allergies. Left-handed people are more likely than right-handed people to be numbered among the ranks of gifted artists, musicians, and mathematicians. Heritability makes about a 24% contribution to the likelihood of being right- or left-handed.

F. Split-Brain Experiments: How Many Brains Do You Have?

A number of people with severe cases of epilepsy have split-brain operations in which much of their corpus callosum is severed. The purpose of the operation is to confine seizures to one hemisphere of the cerebral cortex rather than allowing a neural tempest to reverberate. Split-brain operations do seem to help people with epilepsy. As reported by pioneering brain surgeon Joseph Bogen, each hemisphere may have a "mind of its own." Another pioneer of split-brain research, Michael Gazzaniga, found that people with split brains whose eyes are closed may be able to verbally describe an object such as a key when they hold it in one hand, but not when they hold it in the other hand.

The discrepancy between what is felt and what is said occurs only in people with split brains. Even so, people who have undergone split-brain operations tend to lead largely normal lives. And for the rest of the people, the two hemispheres work together most of the time.

III. The Endocrine System

The body has two types of glands: glands with ducts and glands without ducts. A duct is a passageway that carries substances to specific locations. A number of substances secreted by ductless glands have effects on behavior and mental processes. The ductless glands make up the endocrine system, and they release hormones into the bloodstream. Hormones are then picked up by specific receptor sites and regulate growth, metabolism, and some forms of behavior. Bodily mechanisms measure current levels; when these levels deviate from optimal, they signal glands to release hormones. The maintenance of steady states requires feedback of bodily information to glands. This type of system is referred to as a negative feedback loop.

A. The Pituitary and the Hypothalamus

The pituitary gland lies below the hypothalamus. Although the pituitary is only about the size of a pea, it is so central to the body's functioning that it has been dubbed the "master gland." The anterior (front) and posterior (back) lobes of the pituitary gland secrete hormones that regulate the functioning of many other glands. Growth hormone regulates growth of muscles, bones, and glands. Prolactin regulates maternal behavior in lower animals such as rats and stimulates production of milk in women. As a water conservation measure, vasopressin (also called antidiuretic hormone) inhibits production of urine when the body's fluid levels are low. Oxytocin stimulates labor in pregnant women and is connected with maternal behavior (cuddling and caring for young) in some mammals.

The hypothalamus secretes a number of releasing hormones, or "factors," that stimulate the pituitary gland to secrete related hormones.

B. The Pineal Gland

The pineal gland secretes the hormone melatonin, which helps regulate the sleep-wake cycle and may affect the onset of puberty. Melatonin may also be connected with aging. In addition, it appears that melatonin is a mild sedative, and some people use it as a sleeping pill.

C. The Thyroid Gland

The thyroid gland could be considered the body's accelerator. It produces thyroxin, which affects the body's metabolism—the rate at which the body uses oxygen and produces energy. Some people are overweight because of hypothyroidism, a condition that results from too little thyroxin. Thyroxin deficiency in children can lead to cretinism, a condition characterized by stunted growth and mental retardation. People who produce too much thyroxin may develop hyperthyroidism, which is characterized by excitability, insomnia, and weight loss.

D. The Adrenal Glands

The adrenal glands, located above the kidneys, have an outer layer, or cortex, and an inner core, or medulla. The adrenal cortex is regulated by the pituitary hormone ACTH (adrenocorticotrophic hormone). The adrenal cortex secretes hormones known as corticosteroids, or cortical steroids. Epinephrine and norepinephrine are secreted by the adrenal medulla. Epinephrine, also known as adrenaline, is manufactured exclusively by the adrenal glands, but norepinephrine (noradrenaline) is produced elsewhere in the body. The sympathetic branch of the autonomic nervous system causes the adrenal medulla to release a mixture of epinephrine and norepinephrine that helps arouse the body to cope with threats and stress.

E. The Testes and the Ovaries

The testes and ovaries also produce steroids, among them testosterone and estrogen. (Testosterone is also produced in smaller amounts by the adrenal glands.) About six weeks after conception, the male sex hormone testosterone causes the male's sex organs to develop.

During puberty, testosterone stokes the growth of muscle and bone and the development of primary and secondary sex characteristics. Primary sex characteristics are directly involved in reproduction and include the increased size of the penis, sperm-producing ability of the testes. Secondary sex characteristics such as the presence of a beard and a deeper voice, differentiate males from females but are not directly involved in reproduction. The ovaries produce estrogen and progesterone as well as small amounts of testosterone. Estrogen fosters female reproductive capacity and secondary sex characteristics such as accumulation of fatty tissue in the breasts and hips. Progesterone stimulates growth of the female reproductive organs and prepares the uterus to maintain pregnancy.

Steroids, Behavior, and Mental Processes

Steroids increase muscle mass, heighten resistance to stress, increase the body's energy supply by signaling the liver to release glucose into the bloodstream. The steroid testosterone is connected with the sex drive in both males and females. Anabolic steroids have been used, sometimes in tandem with growth hormone, to enhance athletic prowess. They are also connected with self-confidence and aggressiveness. Anabolic steroids are generally outlawed in sports.

IV. Evolution and Heredity

In 1871 Darwin published The Descent of Man, which made the case that humans, like other species, were a product of evolution. He argued that the great apes (chimpanzees, gorillas, and so on) and humans were related and shared a common primate ancestor. The concept of a struggle for existence lies at the core of Darwin's theory of evolution.

Which species prosper and which fade away are determined by natural selection; that is, species that are better adapted to their environment are more likely to survive and reproduce. Biology serves as the material base for people's behaviors, emotions, and cognitions. Natural variations from individual to individual, along with sudden changes in genes called mutations, lead to differences among individuals, differences which affect the ability to adapt to change. Those individuals whose traits are better adapted are more likely to survive.

A. Evolutionary Psychology: Doing What Comes Naturally

The concepts of adaptation and natural selection have also been applied to psychological traits and are key concepts in evolutionary psychology. Evolutionary psychology studies the ways in which adaptation and natural selection are connected with mental processes and behavior.

One of the concepts of evolutionary psychology is that not only physical traits but also many patterns of behavior, including social behavior, evolve and can be transmitted genetically from generation to generation. Behavior patterns that help an organism to survive and reproduce may be transmitted to the next generation. Such behaviors are believed to include aggression, strategies of mate selection, even altruism. Such behavior patterns are termed instinctive or species-specific because they evolved within certain species.

An instinct is a stereotyped pattern of behavior that is triggered in a specific situation. Instinctive behavior is nearly identical among the members of the species in which it appears. Many psychologists consider language to be "instinctive" among humans. However, even instinctive behavior can be modified to some degree by learning, and most psychologists agree that the richness and complexity of human behavior are made possible by learning.

B. Heredity, Genetics, and Behavioral Genetics

Heredity defines one's nature, which is based on biological structures and processes. Heredity refers to the biological transmission of traits that have evolved from generation to generation. The subfield of biology that studies heredity is called genetics. Behavioral genetics bridges the sciences of psychology and biology. It is concerned with the genetic transmission of traits that give rise to patterns of behaviors. Psychologists are thinking in terms of behavioral genetics when they ask about inborn reasons why individuals may differ in their behavior and mental processes. Heredity appears to be a factor in almost all aspects of human behavior, personality, and mental processes. Examples include sociability, shyness, aggressiveness, thrill seeking, leadership, effectiveness as a parent or a therapist, happiness, even interest in arts and crafts. Heredity is apparently involved in psychological disorders ranging from anxiety and depression to schizophrenia, bipolar disorder, alcoholism, and personality disorders.

C. Genes and Chromosomes

Genes are the most basic building blocks of heredity. It is estimated that the cells within the body contain 20,000 to 25,000 genes. Genes are segments of chromosomes. Chromosomes are made up of strings of genes. Each cell in the body contains 46 arranged in 23 pairs. Chromosomes are large complex molecules of DNA (short for deoxyribonucleic acid), which has several chemical components. The tightly wound structure of DNA was first demonstrated in the 1950s by James Watson and Francis Crick. DNA takes the form of a double helix—a twisting molecular ladder. The "rungs" of the ladder are made up of chemicals whose names are abbreviated as A, T, C, and G.

A group of scientists working together around the globe—referred to as the Human Genome Project—has learned that the sequencing of DNA consists of about three billion DNA sequences spread throughout the chromosomes. Complex psychological traits such as sociability and aggressiveness, are thought to be polygenic—that is, influenced by combinations of genes. A person's genetic code provides his/her genotype—that is, his/her full genetic potential, as determined by the sequencing of the chemicals in his/her DNA. One's phenotype is the manner in which one's genetic code manifests itself because of one's experiences and environmental circumstances. One's genotype provides what psychologists refer to as one's nature. One's phenotype represents the interaction of one's nature (heredity) and one's nurture (environmental influences) in the origins of one's behavior and mental processes.

People normally receive 23 chromosomes from their father's sperm cell and 23 chromosomes from their mother's egg cell (ovum). When a sperm cell fertilizes an ovum, the chromosomes form 23 pairs. The 23rd pair consists of sex chromosomes, which determine whether a person is female or male. When people do not have the normal number of 46 chromosomes (23 pairs), physical and behavioral abnormalities may result. Most persons with Down syndrome, for example, have an extra, or third, chromosome on the 21st pair.

D. Kinship Studies

Kinship studies are ways in which psychologists compare the presence of traits and behavior patterns in people who are biologically related or unrelated to help determine the role of genetic factors in their occurrence. Parents and children have 50% of their genes in common, as do siblings (brothers and sisters). Aunts and uncles related by blood have a 25% overlap with nieces and nephews. First cousins share 12.5% of their genes.

Twin studies

The fertilized egg cell (ovum) that carries genetic messages from both parents is called a zygote. Now and then, a zygote divides into two cells that separate, so that instead of developing into a single person, it develops into two people with the same genetic makeup. Such people are identical, or monozygotic (MZ) twins. If the woman releases two ova in the same month and they are both fertilized, they develop into fraternal, or dizygotic (DZ) twins. Twin studies compare the presence of traits and behavior patterns in MZ twins, DZ twins, and other people to help determine the role of genetic factors in their occurrence. If MZ twins show greater similarity on a trait or behavior pattern than DZ twins, a genetic basis for the trait or behavior is suggested. MZ twins reared apart are about as similar as MZ twins reared together on a variety of measures of intelligence, personality, temperament, occupational and leisure-time interests, and social attitudes.

Adoption Studies