Personality And Health Research Paper

A significant ethical danger in studying the relations of personality and health is that individuals may be unfairly blamed for their health problems. Health and illness are complex states, usually influenced by a wide variety of biological, environmental, and psychosocial factors. Personality—the traits, motives, emotions, and behavioral patterns unique to each individual—is sometimes associated with illness, but it would be a grave oversimplification to think that the individual has direct responsibility for his or her health. On the other hand, individual behavior patterns sometimes do affect health-relevant processes, and so the person, the family, the environment, and the culture can sometimes promote the likelihood of better health.

1.1 Roots In Ancient Greece

The idea that personality and health might be related dates back thousands of years and clearly appears in the writings of Hippocrates, Galen, and their followers. The ancient Greeks, keen observers of the human condition, saw four elements, the so-called bodily humors, as key to both individuality and health. People with a healthy supply of blood might turn out to be sanguine and ruddy. Black bile (or melancholy) might lead to depression and degenerative diseases or cancer. Yellow bile (or choler) could produce a bitter, angry personality and associated diseases, and phlegm is characteristic of a phlegmatic, cold apathy. It is interesting that this scheme of four emotional aspects of personality and health was sufficiently perceptive (or ambiguous) to influence the practice of medicine for 2,000 years. Of course, notions of bodily humors have been discarded. Yet the idea that individuals can be categorized as sanguine, depressed, hostile, and repressed continues to stay with us. Modern personality and health psychology takes these chronic psycho-emotional states and combines them with a more sophisticated understanding of traits to study the associations among personality and health.

1.2 Type A Behavior

Much of the current research interest in personality and health began with theory and research on the Type A Behavior Pattern (Houston and Snyder 1988). Early in the twentieth century, a number of prominent physicians including William Osler began noting that certain men seemed especially prone to angina and coronary heart disease. The at-risk characteristics supposedly included being ambitious, hard-working, dominant, and aggressive. In the 1930s and 1940s, the influence of psychoanalysis on psychosomatic medicine led to numerous speculations about the role of expression and repression of hidden conflicts and anger on the development of heart disease. But it was not until the mid-century writings on Type A behavior that empirical research on this topic began in earnest.

The term ‘Type A’ was chosen by its inventors Meyer Friedman and Ray Rosenman (e.g., Rosenman et al. 1975) because it was a nonconnotative, ‘objective’ designation for a so-called coronary-prone medical syndrome. Type A people were seen as struggling rapidly to accomplish many things in a short period of time, always with a deadline to meet. They were viewed as very competitive, active, and quite hostile or aggressive. Although any person can act this way from time to time, Type A people were seen as constantly driven. Individuals who did not fit this profile were labeled, by default, as Type B. That is, there was thus no independent definition of what a healthy personality style might be.

Although many hundreds of studies have been conducted on Type A behavior, the results have often been mixed and confusing. Although the initial aim was to simplify the concept by choosing a neutral term (‘Type A’) and avoiding related psychological concepts and theories, the characteristics of individuals cannot be adequately explained in such a sterile manner. That is, psychologists soon turned to trying to understand the trait correlates of Type A behavior, the components of Type A behavior, the basis of Type A behavior, the consequences of Type A behavior, and the health aspects of Type A behavior, even beyond coronary disease (Friedman 1992, Houston and Snyder 1988, Miller et al. 1996).

It is now clear that the fast speed and the job-involvement aspects are not important predictors of coronary disease. However, excessive competitiveness and constant hostility seem, for some people in some circumstances, to increase the likelihood of coronary heart disease. It is not yet clear the extent to which these characteristics also raise the risk of all-cause mortality, since there has been insufficient attention to construct validity.

The idea of Type A behavior proved to be an important heuristic one, leading to much empirical research since the 1970s about whether, when, why, and how personality plays a role in health. There are, however, a variety of possible mediating mechanisms.

2. Ways That Personality Is Related To Health: Causal Links

There are many different reasons why personality is associated with health. Understanding the causal pathways is very important, because an interest in this subject usually derives from a desire to improve or promote health. However, it turns out that personality can affect health, that health can affect personality, or that the association between them can be the result of several sorts of artifacts.

2.1 Symptom Reporting And Medical Usage

It is well documented that personality is related to symptom reporting, symptom sensation, and entering the medical care system. Certain people, especially people who are neurotic, with anxiety and stress, are more likely to report symptoms including headache, gastrointestinal distress, and other assorted aches and pains. In other words, if questioned closely, other, non-neurotic people would be experiencing the same symptoms but would not be as likely to talk about them or to report them to their physicians.

It is also the case that people with anxiety and stress are more likely to feel pain and other symptoms. That is, they have a lower threshold for experiencing pain and a lower tolerance for pain. They also may be especially vigilant about bodily sensations (Friedman 1991/2000, Pennebaker 1982).

People with certain psychological traits are also more likely to enter the health care system. These people are generally characterized by such traits as anxiety, depression, dependency, hostility, and moodiness. Women are also generally more likely to seek health care than men. In the extreme, there are people with factitious disorders who seek health care for the rewards that they receive there, including attention, drugs, and shelter (Costa and McCrae 1987, Friedman 1990).

In all of these cases, there are associations between personality and ill health, but the association results from a greater tendency to report and perceive symptoms and seek treatment rather than from a greater incidence of organic disease. In other words, in many cases, certain people are more likely to report being ‘ill,’ but they do not differ much from other people in their incidence of clear organic diseases like heart disease or cancer.

2.2 Disease-Caused Personality Changes

Many associations between personality and health are the result of disease processes that affect health. Certainly any disease with a significant impact on the brain, including Alzheimer’s, Parkinson’s, strokes, syphilis, AIDS, and brain tumors can produce associations between personality and health. Since the changes in personality often become apparent long before the changes in organic brain function are examined and diagnosed, it may appear that the personality is predicting or even causing the disease when the reality is that the disease is changing the personality (Woodward 1998).

In many people, the processes are likely even more subtle and complex. Various illnesses affect the blood supply to the brain, the neurotransmitters in the brain, and communication from other parts of the body to the brain (Woodward 1998). This may result in various associations between personality and health, especially involving depression or other mood changes. For example, low levels of thyroid hormones or lessened oxygen in the blood can cause depression. Importantly, many if not most prescription and nonprescription drugs, being used to treat illness, can affect the brain and thus affect mood and personality, sometimes dramatically, sometimes subtly.

2.3 Temperamental Underlying Third Variables

Some of the associations between personality and health result from a constitutional predisposition underlying certain aspects of personality and certain threats to health. For example, some people’s cardiovascular systems are hyper-reactive, meaning that their blood pressure and heart rate may show marked increases when the individual is under stress. As part of this biology, such people are more likely to be introverted and neurotic (aspects of personality) and are also more likely to suffer diseases such as hypertension. In such cases of an underlying third variable, it is not necessarily the case that efforts to change personality (such as helping an introvert become more sociable) will have any effect on health risk if the underlying physiology is not altered (Eysenck 1991, Friedman 1990, Zuckerman 1991).

Similarly, a hostile temperament, which may result from a combination of genetics, early hormone exposures, and early experiences, could lead to an association between a choleric personality and health risks. Hans Eysenck has argued that such psychobiological systems produce many associations between individual characteristics and disease, but also that the psychobiological characteristics are modifiable, thus producing changes in both personality and disease risk.

2.4 Personality-Caused Illness

Probably the most interesting link between personality and health arises when personality has a fairly direct, causal influence on health. For example, chronic worrying and distress somehow may increase the likelihood of headaches. Since we cannot randomly assign people to personality and do conclusive experiments, inferences about how personality affects health must necessarily be inexact. It is important to employ a wide variety of studies ranging from epidemiological studies of psychological patterns and health down to biological studies of the physiological correlates of personality. The findings must then be pieced together.

3. Explanations Of Personality-Caused Illness

In those cases in which personality affects health, it is useful to distinguish among several primary causal pathways. However, given that these influences occur across time, it is often the case that these mechanisms do not function independently; rather they feed back on and interact with each other (Antonovsky 1979, Bolger and Zuckerman 1995, Friedman 1991 2000).

3.1 Poor Coping

Personality is related to coping, the ways in which an individual contends with stress. New thoughts and behaviors are constantly necessary to adapt to new challenges. With successful coping, the individual interprets and understands the challenge, draws on helpers to address it, and takes appropriate actions. Depending on the challenge, individuals who are depressed, introverted, repressed, unconscientious, vain, or conflicted may be less successful in bringing to bear necessary psychological, social, and behavioral resources (Aspinwall and Taylor 1997, Friedman 1991/2000).

A common issue involves relying on unproductive psycho-emotional ways of coping when a behavioral action could be used to solve the problem or source of stress. For example, a student might go party rather than study to deal with the challenge of an upcoming major exam. On the other hand, an individual might attempt to change the unchangeable. For example, after injury or loss, a person may need to find psychological ways of dealing with his or her new situation in order to cope effectively. Poor coping can lead either to unhealthy behaviors or unhealthy physiological reactions, or both (Maddi and Kobasa 1984, Pennebaker 1995).

3.2 Unhealthy Behaviors

It is well documented that certain personalities in certain situations are more likely to engage in unhealthy behaviors such as smoking, drinking, drug abuse, fast driving, fighting, and so on. Certain people are also less likely to engage in prophylactic measures such as using sunscreen, wearing seat belts, using condoms, brushing teeth, regularly washing hands, and so on. Some individuals are more or less likely to visit physicians, keep follow-up appointments, follow medical regimens, or have screening tests. Since these associations usually depend on at least several factors about the individual, the situation, and the unhealthy activity, it is not usually possible to offer simple, broad generalizations.

3.3 Stress And Physiological Reactivity

Psychophysiological mechanisms often come to mind when one first considers links between personality and health. This is the idea that personality and stress directly impact internal physiological processes, thus causing disease. A more general view here is that the disruption upsets bodily homeostasis, thus allowing disease to take hold.

Much of this work dates back to Walter Cannon’s writings on the ‘fight-or-flight’ response in which catecholamines are released and affect organs and systems throughout the body. Catecholamines (such as epinephrine) can affect processes from heart rate and blood pressure to the metabolism of fats. More recently, it has been established that the immune system is likewise affected by stress hormones. It is well documented that certain people are more likely than others to react to challenge with significant psychophysiological stress reactions (Denollet 1998, Friedman 1992, Kiecolt-Glaser et al. 1998, Zuckerman 1991).

3.4 Tropisms: Seeking Unhealthy Situations

One of the least understood but most intriguing sources of the link between personality and disease involves unhealthy situations. Some individuals gravitate towards healthier and health-promoting psychosocial environments, while other individuals find themselves pulled towards unhealthy environments. In these instances, it is not the case that personality leads directly to unhealthy behaviors or to unhealthy psychophysiological reactions. Rather, the personality facilitates entering the unhealthy situation.

Some of this motivation is derived from genetics, temperament, and early experiences. Other motivation arises from family experiences and consistent peer pressure. People who are impulsive, hostile, psychologically unstable, and have related difficulties in self-regulation are more likely to find themselves in environments that promote smoking, alcoholism, drug abuse, interpersonal violence, promiscuity, and risk-taking hobbies. Furthermore, neuroticism tends to predict to negative life events; in other words, neurotic people are more likely to experience objectively more stressful events (Taylor et al. 1997). Others, who are well socialized, conscientious, and agreeable, are more likely to wind up well educated and surrounded by mature adults. Note that someone who was genetically predisposed to alcoholism or other addiction is very unlikely to develop the health-impairing problem if raised in and exposed to very healthy psychosocial environments.

4. Disease-Prone Traits

Although studies rarely assess a variety of personality traits, a variety of likely mediating mechanisms, and a variety of health outcomes, efforts to piece together the results of different types of research yield a sense of which characteristics are more or less likely to lead to good health.

4.1 Extraversion

Various theories suggest that extraversion and its components of dominance, sociability, and talkativeness should be related to good health, but the evidence is sparse. It is certainly the case that people who are well integrated into their communities, have friends, and have good relations with others are more likely to be healthier. Such social support is not, however, necessarily derived from being highly extroverted. Closer analysis reveals many people who are not especially extroverted but who have important, stable social relationships.

4.2 Optimism

Derived from popular notions of the importance of ‘positive thinking,’ a good deal of research has examined whether optimism is related to health. Optimism is the general expectation that good things will happen. To the extent that optimists may try harder when challenged, take better care of them- selves, and cope better with the adversity of illness, optimism may sometimes be a characteristic of good health. For example, dispositional optimists tend to have a faster rate of recovery from coronary bypass surgery (Aspinwall and Taylor 1997, Scheier et al. 1989). Faster recovery with a positive outlook may also be a result of improved immune system functioning as stress is reduced (Kiecolt-Glaser et al. 1998).

However, there are also many reasons why optimism is not health protective and may be health damaging. The other side of the coin of optimism is ‘optimistic bias’ or unrealistic optimism (Weinstein 1984). This bias involves the faulty belief that negative events are less likely to happen to the optimist than to others. For example, one might agree that ‘people’ who smoke and who do not eat fruits and vegetables are more likely to get cancer, but not worry at all whether the relationship applies to oneself. Importantly, someone who is too optimistic may not take precautions or may not cooperate fully with medical treatment (Peterson et al. 1998, Tennen and Affleck 1987).

4.3 Neuroticism, Hostility, Depression

It is very well established that individuals who are more anxious, hostile, or depressed are more likely to report being ill and seek medical care. Much research controversy has arisen over the extent to which and the circumstances under which these are causal mechanisms, with these personality traits causing disease. As noted, such individuals are more likely to feel, report, and treat symptoms. It is also often the case, however, that these symptoms result from true organic disease, promoted by psychological causes.

The best evidence concerns hostility, which was first investigated in an attempt to better understand coronary proneness. Chronic hostility (and chronic arousal of the sympathetic nervous system) raises the risk of disease and interferes with recovery (Miller et al. 1996).

There is also mounting evidence of the role of chronic depression in increasing the likelihood of development of various illnesses. Depression is associated with cortisol dysregulation, which is also linked to disruptions in lipid metabolism. It is not yet clear whether this is closely tied to hostility or is a different mechanism.

The aspect of neuroticism that is characterized by alienation, anxiety, and pessimism can produce various unhealthy effects. Such people can turn to substance abuse or overeating in an attempt to regulate their anxious states. They may develop insomnia. They may avoid interpersonal assistance and medical assistance. This may lead to a cycle of increasing anxiety, isolation, and health problems. On the other hand, someone whose neuroticism leads to a vigilance about avoiding threats, attending to symptoms, and cooperating with treatment may gain some health benefits.

Some researchers focus more generally on chronic negative affect, which includes both emotional distress like anxiety and depression as well as social interaction such as introversion, alienation, and the inability to express oneself to others. This distressed personality is sometimes referred to as ‘Type D’ (Denollet 1998).

4.4 Conscientiousness And Impulsivity

With significant attention to variables such as chronic hostility and Type A competition, it is perhaps not surprising that certain key aspects of a healthy personality have not been uncovered until recently. There is, however, rapidly growing evidence that conscientiousness and a lack of impulsivity is a key characteristic of healthy individuals. Conscientious people, who are prudent, planning, persistent, and dependable, tend to succeed in various life tasks, including staying healthy.

One major study of individuals who have been followed from childhood until their deaths in adulthood, conducted by Howard Friedman and his colleagues, has repeatedly found evidence that conscientiousness is the key personality predictor of longevity. Interestingly, the protective effect of conscientiousness does lessen the likelihood of death from accident and injury, but it is also protective against death from diseases such as cardiovascular disease and cancer. Conscientiousness does not seem to have its effects simply through one or two behavioral pathways, but rather it seems to affect a whole host of threats to health (Friedman et al. 1995). This finding of the importance of conscientiousness has been confirmed by subsequent studies by others which show that conscientiousness is associated with healthy behaviors, better self-care, better cooperation with medical treatment, and risk avoidance.

4.5 Repression, Lack Of Verbalization, And Lack Of Disclosure

It has long been recognized that among the many people who seem disease-prone, not all are hostile, impulsive, or neurotic. There are some who are too stoic, apathetic, and unemotional. These phlegmatic people, who look cool, calm, and sluggish, may be consciously or unconsciously suppressing or repressing inner conflict or feelings. This phenomenon is not well understood, but such people may appear tense or anxious upon close observation, although they assert friendliness and cooperation. Such people are often alienated or feel powerless.

A behavior pattern involving a repressive or denying coping style, lack of emotional expression, stress and anxiety, and helplessness hopelessness is sometimes referred to as ‘Type C.’ A similar pattern in which individuals are unable to express emotion and seem at higher risk of disease progression is sometimes studied under the rubric of ‘alexithymia.’ There is speculation that such conditions promote the development or the progression of some cancers, but the relationship of this constellation of traits to other illnesses has not been much studied.

James Pennebaker has proposed that inhibition is a form of physiological work that increases the risk of illnesses. On the other hand, people who can talk about emotional experiences seem to set in motion processes to better health. Verbalizing may contribute to cognitive assimilation of stress and/or help people understand and organize their experiences. It may also often lead to increases in levels of social support.

5. Self-Healing Personality

Although most theory and research has focused on disease-prone personalities, it is equally important to examine the characteristics that are associated with good health and longevity. These characteristics are not necessarily simply the absence of the disease-prone traits.

The ‘Self-healing personality’ refers to a healing emotional style involving a match between the individual and the environment, which maintains a physiological and psychosocial homeostasis, and through which good mental health promotes good physical health. The term was proposed by psychologist Howard vs. Friedman in his 1991 book of the same name (Friedman 1991 2000). Self-healing, emotionally balanced people may be calm and conscientious but are responsive, energetic, curious, secure, and constructive. They are also people one likes to be around. People with self-healing personalities have better odds of a longer, healthier life.

Self-healing individuals often appear enthusiastic, and they may move in smooth, synchronized styles. Their relative lack of emotional conflict and anxiety may result in fewer speech disturbances, modulated voice tones, and an emotional consistency. Contrary to some common wisdom, a self-healing style does not involve a lack of challenge. In fact, self-healing individuals are often found in challenging, socially valued careers. They may enjoy stimulating travel because they are curious and enjoy exploration.

Interestingly, self-healing personalities are often similar to the mentally healthy orientations that have been described by humanistic psychologists (Antonovsky 1979, Csikszentmihalyi 1991, Maddi and Kobasa 1984). Humanistic psychologists such as Abraham Maslow thought that they were describing mental health when they wrote about self-actualized people who are concerned with issues of beauty, ethics, and justice. But it turns out that this spontaneous, positive, growth orientation is also associated with physical health. Indeed, it is a false dichotomy to divide physical and mental health in these ways. This positive process is sometimes also referred to by Mihaly Csikszentmihalyi as ‘flow.’

5.1 Efficacy And Hardiness

Work by Salvador Maddi and Suzanne Ouellette Kobasa helped provide a framework for thinking about staying healthy in the face of challenge. They call these personality attributes hardiness. First, they suggest that a healthy personality maintains a sense of control. This is not necessarily a wild sense of optimism, but rather a sense that one can control one’s own behaviors. Second, there is a commitment to something that is important and meaningful in their lives. This may also involve values and goals. Third, hardy people welcome challenge. For example, they may view change as an exciting challenge to their growth and development. Efficacy and hardiness generally involve a productive orientation, a sense of mastery, and a zest for life (Bandura 1997).

5.2 Salutogenesis

The medical sociologist Aron Antonovsky (1979) proposed an approach termed ‘salutogenesis,’ a theory of how and why certain people stay healthy. What is interesting about this approach is that it does not postulate a sense of control as a primary or even necessary characteristic of a healthy personality. Rather, Antonovsky refers to a ‘sense of coherence’ which is central to successful coping with challenge. Coherence means that the world is understandable, manageable, and meaningful. In this scheme, someone who believes that nature or a divine force is in charge of the world could have a very healthy orientation, even though the individual does not have a personal sense of control. There is so far only scattered evidence to support this promising approach.

6. Conclusion

In sum, certain individuals are predisposed to be vulnerable to physiological, psychosocial, and interpersonal disturbances and stresses due to a combination of genetics, emotional temperament, and early experiences. When such individuals seek out or are placed in environments that bring out, create, or exacerbate these disturbances, chronic stress patterns may result. These may be accompanied by unhealthy behaviors such as substance abuse. When these individuals then encounter microbes, toxins, or excesses such as high-fat diets, the likelihood of illness increases significantly. On the other hand, other individuals have developed processes of self-healing, which build on themselves as healthier emotional styles, physiological reactions, behaviors, and social interactions become mutually reinforcing.

Overall, there is good evidence that individual characteristics are sometimes tied to health through the biological, psychological, and social aspects of personality, and there is some evidence that focused interventions can have short-term salutary effects on health. But relatively little is understood about the precise causal inter-relations of physiology, coping, social influence, and health behavior, and little is known about the extent to which a focus on personality alone can have substantial effects on various significant health outcomes. It will be important in the future to unravel how these key processes come together and affect each other, both across short time periods and across the lifespan.

Bibliography:

1. Antonovsky A 1979 Health, Stress and Coping. Jossey-Bass, San Francisco
2. Aspinwall L G, Taylor S E 1997 A stitch in time: Self-regulation and proactive coping. Psychological Bulletin 121: 417–36
3. Bandura A 1997 Self-efficacy: The Exercise of Control. Freeman and Co., New York
4. Bolger N, Zuckerman A 1995 A framework for studying personality in the stress process. Journal of Personality and Social Psychology 69(5): 890–902
5. Costa P T, McCrae R R 1987 Neuroticism, somatic complaints, and disease: Is the bark worse than the bite? Journal of Personality 55: 299–316
6. Csikszentmihalyi M 1990 Flow: The Psychology of Optimal Experience. HarperCollins, New York
7. Denollet J 1998 Personality and coronary heart disease: The Type-D Scale-16 (DS16). Annals of Behavioral Medicine 20(3): 209–15
8. Eysenck H J 1991 Smoking, Personality, and Stress: Psychosocial Factors in the Prevention of Cancer and Coronary Heart Disease. Springer-Verlag, New York
9. Friedman H S (ed.) 1990 Personality and Disease. Wiley and Sons, New York
10. Friedman H S 1991/2000 Self-healing Personality: Why Some People Achieve Health and Others Succumb to Illness. Henry Holt, New York (www.Iuniverse.com), 2000
11. Friedman H S (ed.) 1992 Hostility, Coping, and Health. American Psychological Association, Washington, DC
12. Friedman H S, Booth-Kewley S 1987 The ‘disease-prone personality’: A meta-analytic view of the construct. American Psychologist 42: 539–55
13. Friedman H S, Tucker J S, Schwartz J E, Tomlinson-Keasey C, Martin L R, Wingard D L, Criqui M H 1995 Psychosocial and behavioral predictors of longevity: The aging and death of the ‘Termites.’ American Psychologist 50: 69–78
14. Houston B K, Snyder C R 1988 Type A Behavior Pattern: Research, Theory and Intervention. Wiley, New York
15. Kiecolt-Glaser J K, Page G G, Marucha P T, MacCallum R C, Glaser R 1998 Psychological influences on surgical recovery. American Psychologist 53: 1209–18
16. Maddi Salvatore R, Kobasa S C 1984 The Hardy Executive: Health Under Stress. Dow Jones-Irwin, Homewood, IL
17. Miller T Q, Smith T W, Turner C W, Guijarro M L, Hallet A J 1996 Meta-analytic review of research on hostility and physical health. Psychological Bulletin 119: 322–48
18. Pennebaker J W (ed.) 1995 Emotion, Disclosure, and Healing. American Psychological Association, Washington, DC
19. Pennebaker J W 1982 The Psychology of Physical Symptoms. Springer-Verlag, New York
20. Peterson C, Seligman M E P, Yurko K H, Martin L R, Friedman H S 1998 Catastrophizing and untimely death. Psychological Science 9: 127–30
21. Rosenman R H, Brand R J, Jenkins C D, Friedman M, Straus R, Wurm M 1975 Coronary heart disease in the Western Collaborative Group Study: Final follow-up experience of 81/2 years. Journal of the American Medical Association 233: 872–7
22. Scheier M F, Magovern G J, Abbott R A., et al 1989 Dispositional optimism and recovery from coronary artery bypass surgery: The beneficial effects on physical and psychological well-being. Journal of Personality and Social Psychology 57: 1024–40
23. Taylor S E, Repetti R L, Seeman T 1997 Health psychology: What is an unhealthy environment and how does it get under the skin? Annual Review of Psychology 48: 411–47
24. Tennen H, Affleck G 1987 The costs and benefits of optimistic explanations and dispositional optimism. Journal of Personality 55: 377–93
25. Weinstein N D 1984 Why it won’t happen to me: Perceptions of risk factors and susceptibility. Health Psychology 3: 431–57
26. Wiebe D J, Smith T W 1997 Personality and health: Progress and problems in psychosomatics. In: Hogan R, Johnson J, Briggs S (eds.) Handbook of Personality Psychology. Academic Press, San Diego, CA, pp. 891–918
27. Woodward J L 1998 Dementia. In: Friedman H S (ed.) Encyclopedia of Mental Health. Academic Press, San Diego, Vol. 1, pp. 693–713
28. Zuckerman M 1991 Psychobiology of Personality. Cambridge University Press, New York