Chapter 2: Biology and Human Potential

Evolution: Adaptation through Natural Selection

Mostly Nature

As we move along in the book, I will frequently try to relate the current material to major themes described in Chapter 1. I will ask you to periodically recall Maslow’s pyramid of human needs, the video of the Nukak tribe in the rainforest, and the transformation of New York City over the course of two centuries. Despite the inventions and technological innovations of the relatively recent past, there are still people living the nomadic Stone-Age lifestyle which characterized the human condition for almost our entire time on this planet. What made it possible and what was necessary in order for this transformation to occur?

We share many basic needs and behavior with other species. We all inhabit a planet replete with edible foods, predators, and potential sexual partners. Unless we continue to eat adequately, successfully avoid and/or escape from predators, and mate, we will not survive as individuals or as a species. In order to eat, survive, and reproduce, we need to be able to sense food and danger, identify receptive mates, and respond in an adaptive manner.

The Mostly Nature section addresses how our physical structure impacts upon our ability to survive and realize our potential as individuals and a species. In Chapter 1, we reviewed how our contemporary approach to psychology integrated the interests and goals of the early schools. This will be reflected in the chapters which comprise this section of the book. The earliest school, eventually named structuralism, was primarily concerned with our internal world consisting of sensations, images, and feelings. In Chapter 3, we will review the structure and function of our sense organs for external stimuli (e.g., our eyes for vision, ear for hearing, etc.) addressing how experience results in combining stimulus elements into meaningful patterns (i.e., gestalts). The functionalist school was concerned with how our internal world enabled us to adapt to our external world. Chapter 4 reviews how we process and eventually respond to internal stimuli (e.g., hunger for food deprivation, thirst for water deprivation, pain, etc.).

The current chapter reviews the evolutionary processes and hereditary mechanisms which resulted in the structure of the human body. We will see how our brain and nervous system transmit and interpret sensory information relaying it to parts of the body capable of responding. All of the inferences and conclusions drawn regarding how our internal processes enable us to adapt to our environment are based upon behavioral observations. At times you may wonder why so much detail is provided regarding human anatomy. You were expecting a course in psychology, not biology. I have done my best to emphasize those parts of the nervous system crucial to our survival and achievements as a species. We will start with a discussion of how human anatomy evolved.

Darwin’s Theory of Natural Selection

Between December of 1831 and October of 1836, Charles Darwin took one of the most important geographic and intellectual journeys in recorded history. On this voyage he collected fossils and observed many forms of wildlife. Upon returning to England and examining his evidence, Darwin detected patterns in variations of animals seemingly related to environmental conditions. For example, he observed that the size and form of a species of birds’ beaks appeared related to the types of available foods (see Figure 2.1). Eventually he published The Origin of Species (1859), arguably the most influential book in the history of biological (if not all) science.

Figure 2.1 Darwin’s finches

Darwin was familiar with the selective breeding practices of farmers designed to result in improved stocks. He reasoned that a similar selective process could occur as the result of natural causes (i.e., natural selection). That is, if environmental factors resulted in some animals having an adaptive advantage relative to others, those animals would be more likely to survive long enough to reproduce. With respect to the birds, those possessing the type of beak best suited to eating the available food type would be able to consume more food and be more likely to survive. Similarly, if some animals possessed a characteristic resulting in their being more attractive to potential mates, they would be more likely to breed (Figure 2.2).

Figure 2.2 Sexual selection

The most controversial aspects of Darwin’s theory of natural selection relate to human evolution. The implication is that our physical structure, and therefore our behavioral potential, is the result of a natural process related to arbitrary environmental factors. Darwin did not at first relate natural selection to human evolution but eventually did so in his later publication, The Descent of Man (1871). It was not until a century later that a substantial number of fossils suggesting very gradual changes in human structure were discovered. This provided physical evidence for human evolution through natural selection.

You might be wondering if the human being is still evolving. In fact, there are several examples of relatively recent adaptive biological modifications apparently resulting from environmental changes. For example, tens of thousands of years ago, humans started moving to higher altitudes. DNA samples from cultures with a history of living in the mountains included genes impacting upon the amount of hemoglobin in the blood (NY Times, May 30, 2013). Individuals with these genes would be better able to cope with the low oxygen levels characteristic of high altitudes. For example, Tibetans tend to have broader arteries and capillaries than nearby Chinese populations living in low-lying areas. These broader vessels permit greater blood flow and a corresponding increase in delivery of oxygen to the cells of the body.

Heredity and Genetics

Darwin did not possess our current knowledge of the mechanisms involved in heredity, the transfer of characteristics from parent to child. He thought that half the characteristics of each parent were combined and transmitted to the next generation. This was a plausible hypothesis given the obvious similarities and differences between parents and offspring. However, if nothing else was involved, natural selection could not occur. Darwin had specified a process for selection but not for the potential variation in genes necessary for change to occur over time. The possibility of genetic mutation was yet to be discovered.

Although occurring at about the time that Darwin published The Origin of the Species (1859), Gregor Mendel’s genetic research with peas did not attain influence until early in the twentieth century. Mendel’s findings enabled Theodor Boveri (1904) to demonstrate the role of chromosomes (tiny threads contained within a cell’s nucleus) in heredity. Cells are the basic building blocks for plants and animals. The human body is comprised of trillions of cells. In 1910, Thomas Hunt Morgan, studying heredity in the fruit fly, demonstrated that genes were located on chromosomes in the cell nucleus. Genes are the basic units of heredity and ordinarily occupy constant positions on chromosomes. Genes are comprised of DNA (deoxyribonucleic acid), which includes all the information required for cell replication. Nearly every cell in the body has the same DNA. In 2003, the Human Genome Project reported that the entire human genome (i.e., all the genetic information characteristic of our species) consisted of approximately 20,000 genes. We now know that most genetic variation results from mutation, a permanent chemical change in the composition of a gene’s DNA. Mutations are rare, and ordinarily provide no adaptive advantage. Thus, it is not surprising that evolution through natural selection is very slow, taking millions of years in humans. First, an adaptive mutation must occur in a member of a species which must be fortunate enough to survive and successfully mate. Then, multiple generations would be required for this difference to show up in a significant number of other surviving individuals. It should be emphasized that the adaptive value of a mutation depends upon the environment in which it occurs (Figure 2.3). For example, one third of the indigenous inhabitants of Sub-Saharan Africa carry the gene for sickle cell disease. This gene increases immunity to malaria, making it adaptive in Africa while being maladaptive elsewhere.

Figure 2.3 Explanation of evolution

Genotypes and Phenotypes

The inherited instructions contained within an individual’s genes are referred to as its genotype. Your DNA includes all the information required to create an individual with your exact physical characteristics, susceptibility to specific diseases, and even some of your temperament. At fertilization, humans inherit 46 chromosomes, 23 apiece from our biological mothers and fathers. The 23rd male chromosome determines the sex of the child. This is because females carry two of the same sex chromosome (called X) whereas males carry one X and one Y (male) chromosome (see Figure 2.4). The chromosome pairs and DNA sequences are structurally similar. Traits are passed on from generation to generation through the DNA contained in genes on these chromosomes.

Figure 2.4 The Y-chromosome

Mendel discovered that when crossing plants with different characteristics (e.g., white or purple) the result was not a blend. Rather, one of the initial characteristics would occur (e.g., the next generation would be purple). Mendel referred to this as a dominant trait and to the other as a recessive trait. For example, in humans brown hair is dominant over red hair. This means that if a child inherits a brown gene from one parent and red gene from the other parent, her/his hair will be brown. However, since that child carries both gene colors, it is possible that offspring will inherit red hair. Traits can skip generations.

The observable physical and behavioral characteristics of a species are referred to as its phenotype. We saw in Chapter 1 that complex human behavior is the result of an interaction between hereditary and environmental variables. It is also true that environmental factors interact with genetic factors with regard to physical traits. For example, an individual’s height and weight can be influenced by nutrition, infection, and other variables.

Human Evolution

It is estimated that the universe is 13.8 billion years old and that the earth is 4.54 billion years old (Dalrymple, 2001). The human being is the most complicated animal on earth. Physical evidence suggests life in the form of simple cells first appeared 3.6 billion years ago. Half a billion years ago, animal life emerged from the sea. Over time, closer approximations to a modern human being appeared. In terms of biology’s family tree, humans are considered primates, a branch including apes, monkeys, and lemurs for which we have discovered fossils dating back approximately 55 million years. The human being’s closest existing primate “relatives”, chimpanzees, bonobos, and gorillas, diverged approximately four to six million years ago. Fossilized evidence suggests the first bipedal animals (i.e., standing on two legs) appeared approximately seven million years ago with the earliest documented evidence for humans (i.e., members of the biological genus “Homo”) dating back approximately 2.3 million years. It is at this time that we observe the first indication of use of stone tools by Homo hablis. This and other transformational events are depicted in Figure 2.5. Note that the brains of early humans were comparable in size to those of chimpanzees and gorillas. This size more than doubled over the course of 2 million years until the appearance of Homo sapiens (modern humans) approximately 200,000 years ago.

Figure 2.5 Human timeline

None of the transformational discoveries listed in Figure 2.5 was inevitable. Evolutionary biologist Jared Diamond (2005) wrote a wonderful Pulitzer prize-winning book tracing the history of the human being leading up to and after the last ice age, approximately 13,000 years ago. He describes how features of the climate and environment impacted on the course of development of humans on the different continents and why some cultures eventually became dominant over others. For most of our time on earth, variations of the human species survived as nomadic small bands of hunter/gatherers in Africa. Diamond (2005, 36-37) describes fossilized evidence that humans migrated to Southeast Asia approximately 1 million years ago, with Homo sapiens reaching Europe ½-million years ago. Existing evidence suggests that modern humans reached the Americas between 14,000 and 35,000 years ago. Diamond (2005, 87) provides an overview of the causal factors impacting upon the human condition. Geographic and climatic conditions affecting the availability of wild plants and animals determined the possibility of development of agriculture and animal domestication. Localized food production enabled establishment of more permanent residences and larger communities. Food storage permitted surpluses, freeing people from survival demands on a day-to-day basis. This resulted in development of new “occupations” and technologies, dramatically altering the human condition.

My students readily admit that where they were born was an extremely important yet arbitrary event impacting upon the course of their life. If they were born and lived in the rain forest they would not be prepared to attend college. Alternately, if at their present age they were dropped in the rain forest without modern technologies or assistance from natives, they would probably not survive.

The terms “human evolution”, “ human condition”, and “human potential”, must be carefully analyzed to be most meaningful. Our potential as individuals and as a species starts with the physical structure (i.e., anatomy) that evolved through the process of natural selection. The Mostly Nature section of this book describes how our anatomy permits us to sense specific sources of physical stimulation originating from outside (Chapter 3) and within (Chapter 4) our bodies. The current chapter describes how the anatomy of our brain and nervous system enables us to process and coordinate this physical stimulation and transmit information to those parts of our body capable of responding. Our anatomy places limits on the types of physical information we are able to sense and the types of responses we are able to make. These limitations meant that we were not always capable of surviving and reproducing under all the geographic and climatic conditions that existed on earth. Our physical structure, however, included the potential to touch, manipulate, and change the planet. The development of tools and technologies magnified this capability, eventually resulting in transformative changes in our environmental conditions. We refer to the interface between our physical structure and environment as the human condition. Over the millennia, humans acquired the capability of surviving anywhere on this planet, traveling to the moon, and exploring the universe. The fascinating story of the past, present, and future potential of our species starts with an understanding of human genetic potential

Human Genetic Potential

When I was a child I faithfully watched the Superman TV show. I would play with my friends, pretending to be Superman, and try to leap off a step while wearing my cape (actually a towel). Despite doing my best to imitate my hero I never took off. I wasn’t a bird or Superman. Flying was beyond my genetic potential. We saw that the human brain significantly increased in size over the course of evolution. A convenient way of considering what constitutes the genetic potential for human behavior is to examine the human motor homunculus (little person). This is a representation of the amount of “brain space” in the cortex allotted to different parts of our body for acting upon our environment (see Figure 2.6). Consistent with the distinctive human DNA described in Figure 1.2, a disproportionate amount of the cortex is allocated to organs related to speech (lips, jaw, tongue, and voice box) and to the hands (particularly the thumb). The ability to manipulate our facial muscles, tongue, and larynx provides the potential to emit an enormous variety of vocalizations. Initial attempts to teach chimps to speak (Hayes and Hayes, 1952) were unsuccessful, primarily due to limitations in the use of these body parts. Our ability to manipulate our fingers and thumbs to form the “precision grip” enables us to grasp and hold objects of different sizes and shapes.

Figure 2.6 The motor homunculus

Millions of years of evolution resulted in an animal with the genetic potential to learn complex behaviors, speak and create tools. This potential took a very long time to emerge. However, once realized, the combination of imagination, communication, and manipulation resulted in humans dominating and changing our planet. Theodosius Dobzhansky (1960), a noted Russian genetic biologist, stated

“Mutation, sexual recombination and natural selection led to the emergence of Homo sapiens. The creatures that preceded him had already developed the rudiments of tool-using, tool making and cultural transmission. But the next evolutionary step was so great as to constitute a difference in kind from those before it. There now appeared an organism whose mastery of technology and of symbolic communication enabled it to create a supraorganic culture. Other organisms adapt to their environments by changing their genes in accordance with the demands of the surroundings. Man and man alone can also adapt by changing his environments to fit his genes. His genes enable him to invent new tools, to alter his opinions, his aims and his conduct, to acquire new knowledge and new wisdom.” The “supraorganic culture” Dobzhansky describes, results from the multiplicative effects of human’s shared imaginings, communications and manipulations. Throughout this book we will seek to understand our potential to transform the world, the human condition and our individual selves. We start with the human genotype.

The Nervous and Endocrine Systems

The Nervous System: Connecting Sensation and Movement

As we consider the human genotype, we will start by providing an overview of the nervous system (see Figure 2.7), those structures which transmit information regarding external and internal stimulation and coordinate behavior.

Figure 2.7 Overview of human nervous system

The central nervous system, consisting of the brain and spinal cord, organizes and interprets information received from the peripheral nervous system and initiates responding. The somatic division of the peripheral nervous system responds to sensory information originating outside the body and stimulates the skin, joints, and skeletal muscles. This type of behavior is often considered voluntary. The autonomic nervous system governs the activity of the smooth muscles and glands internal to the body involved in circulation, respiration, and digestion (see Figure 2.8). This type of activity is often considered involuntary. The sympathetic division results in arousal under stressful or dangerous conditions as the body is prepared for “fight or flight.” The parasympathetic division calms the body upon removal of the stress or danger.

Figure 2.8 The autonomic nervous system

Making the Physical Connections: The Neuron

Even very simple animals require some way of connecting environmental input with behavioral output. Specialized nerve cells called neurons are required to respond to external and internal stimulation (i.e., sensory neurons) and carry information to parts of the body capable of responding (i.e., motor neurons). A third type of cell referred to as an interneuron connects nerve cells to each other. Nervous systems consist of these types of specialized neurons and range in size from a few hundred nerve cells in worms to approximately 100 billion nerve cells in humans. Neurons are capable of transmitting information electrically and chemically. Figure 2.9 portrays the major parts of a neuron. Dendrites are small branches which can connect to nearby neurons. A single axon can extend in length up to about a meter in humans and connect to the dendrites of more distant neurons. For example, a neuron could connect the spinal cord to a foot.

Figure 2.9 The neuron

Nerve cells “fire” (i.e., achieve their electrical action potential) according to an all-or-none principle. That is, either the cell is totally activated or not at all. Increasing the intensity of stimulation does not increase the likelihood of a nerve responding. Rather, it increases the nerve’s rate of firing (i.e., frequency over time). For example, as a lamp becomes brighter, this does not increase the likelihood of a receptor cell in your eye firing. Rather, it increases the frequency with which the receptor cell fires. Nerves can fire at rates as high as a thousand times per second.

Making Chemical Connections: Neurotransmitters

The chemical exchange between neurons occurs at synapses, the small spaces separating the dendrites and axon endings (see Figure 2.10).

Figure 2.10 The synapse

The first nerve cell releases chemical neurotransmitters that can bind with receptors in the second neuron. The exchange can result in excitation or inhibition, depending upon the type of receptor activated. Figure 2.11 lists the major neurotransmitters along with their roles in the body.

Figure 2.11 The major neurotransmitters

Psychoactive drugs can affect mood, thought, and behavior. Most achieve these effects by impacting upon neurotransmitters and synaptic connections. In Chapter 11 (Maladaptive Behavior), we will consider the use of psychoactive drugs in the treatment of depression and schizophrenia.

The Brain

For literal and figurative reasons, it is tempting to refer to the human brain as evolution’s crowning achievement. After all, the brain sits atop our nervous system and enables our most complex overt and covert behaviors. Your thoughts, your feelings, all the complex things you do, would not be possible without this organ housed inside your skull on top of your head.

The human brain is similar in construction to the brains of other mammals but much larger in comparison to the size of our bodies. Without the increase in brain size occurring during human evolution it would not matter if we inherited the physical structures necessary to speak and create tools. This potential would never be realized. Manhattan would still look the same as it did 400 years ago. We are now using our remarkable brain to study itself. The United States government declared the 1990s as the “Decade of the Brain” and much progress has been made in understanding how the brain operates. President Barack Obama of the United States declared “The BRAIN (Brain Research through Advancing Innovative Neurotechnologies) Initiative” in 2013, hoping to advance this knowledge.

There are many ways of describing the brain in terms of its structure (i.e., anatomy) or function (i.e., parts operating together in producing a specific effect). Figure 2.12 shows the major parts of the human brain. The prefrontal cortex is involved in the higher human cognitive functions including attention, perception, thinking, memory, language and consciousness.

Figure 2.12 Human brain

The brain is an adaptive organ connecting sensation with movement. Besides the primary somatosensory area in the parietal lobe, sensory areas include the occipital lobe for vision and temporal lobe for hearing. Besides the primary motor area at the rear of the frontal lobe, motor areas include the brain stem and spinal cord. The rest of the cortex is referred to as association areas and is dedicated to perception and cognition. It is the size and structure of this area which expanded enormously as humans evolved and enabled us to not only survive but to transform the human condition.

A human brain weighs about three pounds and feels “squishy” (something like gelatin). The cerebral cortex covers most of the brain and is comprised of nerve fibers folded in such a manner (called convolutions) to increase the amount of surface area in the total space. There are two symmetrical halves often referred to as the left brain (or hemisphere) and right brain (or hemisphere). The two halves are connected by the corpus callosum, a thick network of nerve fibers enabling the two sides to communicate. The left side of the brain connects to the right side of the body and vice versa. Certain activities appear more characteristic of one side than the other (see Figure 2.13). These distinctions are referred to as lateralization. Despite the different emphases, both sides usually act in concert in carrying out these activities (Toga & Thompson, 2003).

Figure 2.13 Brain lateralization

Most of the expansion in the size of the human brain occurred in the frontal lobe. This part of the brain is involved in self-control, described in Chapter 1, and in abstract thought and problem-solving, described in Chapter 7. The small occipital lobe is dedicated to vision, described in Chapter 3. At the borders of the frontal and parietal lobes is a deep fissure (the central sulcus) where large strips of neural tissue dedicated to sensation (the primary somatosensory cortex) and movement (the primary motor cortex) meet. The temporal lobe is primarily involved with memory and language, described in Chapter 6. The parietal lobe is involved with sensation originating in the skin, muscles, and joints.

The Endocrine System: Hormonal Regulation

The endocrine system consists of ductless glands that secrete hormones (chemical messengers) into the blood stream to maintain homeostasis. It exists in all animals having a nervous system. Like the nervous system, the endocrine system enables communication between different parts of the body.

The endocrine system maintains homeostasis through a series of feedback loops, the most important of which are controlled by the hypothalamus interacting with the pituitary gland. Often, the hypothalamus stimulates the pituitary gland to secrete an activating hormone to another gland. If a signal is transmitted to a gland, indicating low blood levels of its hormone, it secretes additional amounts into the blood stream. Once the optimal level is restored, the gland stops secreting the hormone. In this way the endocrine system plays its critical role in metabolism, growth, sexual development, reproduction, and responding to stress. Figure 2.14 shows the locations of the major glands.

Figure 2.14 Major gland locations

The pituitary gland connects to the base of the hypothalamus and is often referred to as the master gland since it secretes several different hormones impacting upon other glands involved in maintaining homeostasis. Hormones secreted by the pituitary control growth, blood pressure, water balance, temperature regulation, and pain relief. The pineal gland is located at the base of the cortex between the two hemispheres and next to the thalamus. It influences the sleep-wake cycle by secreting the hormone melatonin when stimulated by light. The thyroid gland is located in the neck by the larynx (voice box) and affects metabolism by controlling the rate at which energy is expended. It is one of the glands under the control of the pituitary which secretes thyroid-stimulating hormone (TSH). The pituitary in turn is controlled by the hypothalamus through the release of thyrotropin-releasing hormone (TRH). Humans usually have four parathyroid glands located on the rear surface of the thyroid gland. These control the amount of calcium in the blood and bones. The thymus is located below the thyroid gland in the middle of the chest. It is an important part of the immune system. Damage, such as through contracting the HIV virus, can result in increased susceptibility to infection (e.g., AIDS). The spleen lies toward the bottom of your rib cage and is involved in the removal of red blood cells. The adrenal glands are located on top of the kidneys and through the release of epinephrine (adrenalin) are significantly involved in the body’s “fight-or-flight” response in reaction to danger. The sex glands (ovaries for the female and testes for the male) secrete hormones controlling the development of the reproductive sex organs and secondary sex characteristics (e.g., pubic hair) during puberty.

Self-Control and Biological Psychology

Life is a rat race

In the first chapter, we saw how an experiment conducted with pigeons provided important insights into the self-control process. At the end of each chapter, I will consider implications of the material to achieving your own potential. You might be wondering how this could be possible in a chapter describing human biology. Think of the implications of neuroplasticity. By making environmental manipulations, you can actually “rewire” your brain. What may surprise you is that studies measuring EEGs and MRIs with humans have demonstrated that physical activity affects the brain. It is known that the cortex and hippocampus atrophy in the aged. Experimental studies with other animals (primarily rats and mice) have demonstrated that exercise can actually increase the number of nerve cells in the hippocampus (Brown, Cooper-Kuhn, Kempermann, van Praag, Gage, & Kuhn, 2003; van Praag, H., Shubert, Zhao, & Gage, 2005; Eadie, Redilla, & Christie, 2005).

You are probably aware of the many benefits of exercise. The Public Health Service concluded exercise was one of the most significant ways in which humans could improve their health (Powell, & Paffenbarger, 1985). Consistent aerobic exercise has been found to reduce the likelihood of developing hypertension (high blood pressure), heart disease, type II diabetes, and osteoporosis (thinning of the bones). A good way to understand research is to use the scientific schema I described in Chapter 1. Ask yourself to describe out loud or in writing the question being investigated, the procedures used to investigate the question, the research findings, and the conclusions. It is often helpful to try to imagine yourself as a research subject. I am sure it is easy for you to imagine being on a treadmill or exercise bike with electrodes attached to your head to take EEG readings. That would enable us to observe the effects of independent variables (e.g., duration of exercising, speed, the incline on a treadmill or resistance on a bike) on a dependent variable (e.g., changes in EEG recordings).

In order to obtain more precise data regarding the effects of exercise on the brain, it is necessary to examine the brain itself rather than simply recording EEG or obtaining MRI scans. This type of invasive research can only be conducted on other animals. You may be wondering, how is it possible to study the effects of exercise on rats and mice. Are there rat and mice gyms or health clubs? Do they have swimming pools, bikes, and treadmills? The answer is, sort of (Figure 2.15).

Figure 2.15 Rat in a running wheel.

One study compared the effects of moderate and more intense exercise on two types of learning and on changes in the presence of neural plasticity-related proteins in the hippocampus and amygdala (Liu, Chen, Wul, Kuol, Yu, Huang, Wu, Chuang, & Jen, 2009). Both groups were taught to swim to the end of a water maze and to avoid shock in another apparatus. Then, one group of mice engaged in self-paced wheel running for four weeks and another group received more intense workouts on a treadmill. Both groups were then retested on the two tasks and underwent surgery to assess changes in their brain chemistry. After their four weeks of exercise, both groups improved their performance in the water maze. Only the group given the more intense treadmill workouts improved on the more difficult shock avoidance task. It was determined that this group increased the levels of specific proteins in both the hippocampus and amygdala whereas the self-paced group only experienced increases in the hippocampus. The authors concluded that different exercise routines can differentially affect learning and brain chemistry.

In Chapter 1, I listed examples of the behaviors my students have targeted in self-control projects. Many students are interested in improving their cardiovascular fitness by engaging in aerobic exercises (i.e., involving sustained rhythmic activity). According to the American College of Sports Medicine (ACSM) Physical Activity Guidelines for Americans (Garber, 2011), one should engage in 1-1/4 hours (75 minutes) of intense (e.g., jogging or running) or 2-1/2 hours (150 minutes) of moderate (e.g., brisk walking) exercise per week. You can break up the activity into sessions lasting at least 10 minutes. For example, you could walk briskly for half an hour on five different days, or 50 minutes on three different days, etc.

Usually, for exercise projects, students assess the frequency, duration, and intensity of exercises they would like to perform. In Chapter 5, we will discuss application of learning principles to modify behavior. However, if you would like to try a self-control intervention before then, you could implement an adjusting criterion procedure. In the case of aerobic exercise, start by obtaining baseline data on the frequency, duration, and (when applicable), the intensity of your sessions. If you do not exercise at all, try a minimal amount (e.g., five or ten minutes) on three different days. Once you have stable baseline data, you will be ready to adjust the criterion upward in order to earn a reward. For example, you could require a ten per cent increase (to five and a half or eleven minutes). If you earn the reward three sessions in a row you can increase the criterion by an additional ten per cent, and so on until you reach your ultimate objective. The reward could be something tangible such as a treat (but watch out for the calories!) or money, or an enjoyable activity such as reading, listening to music, playing video games, engaging in social networking, etc. Make sure you leave sufficient time to study for your courses, though. Otherwise you will need to implement an adjusting criterion procedure on study time!