This paper examines the phenomenon of birth order as it particularly relates to only children. Only children are unique in birth order in that they are the first- and last-born children in their families. Various theories of prominent psychologists such as Adler, Freud, Skinner, and Eysenck will be examined in their application to the importance of birth order in personality development. A theory concerning only children will then be presented, dealing chiefly with their difficulty with the labels of introversion and extraversion. Personal observations as well as a proposal for testing the theory will be given.
Procreation has been an essential task for all human beings in order to continue the existence of the species. Before the advent of modern medicine and birth control, common sense would dictate that females would give birth to a large number of children, helping to ensure that at least one would survive to adulthood and thus create children of his or her own. However, as time has passed, humans have become able to control the number of children they have. Many choose to have more than one child, some choose to have none at all. Still others choose to have only one. Whatever the decision, the number and order of birth of human offspring seems to have at least a small effect on their personality development. Only children are special cases and must be looked at in a slightly different manner.
Birth Order and Its Repercussions
Adler and Birth Order
Adler (Weiten, 1998), best known for his theories regarding striving for superiority, was also concerned with the effects of birth order on personality. Adler had a successful older brother, but Adler was weak as a child and thus was most likely affected with the desire to assert himself and prove his worth.
Adler's theory stressed the social aspect of personality development and therefore proposed the possibility of birth order and its significance in the interpersonal relationships of family life. He felt that each position in the order, whether first or last, had distinct characteristics. For example, he hypothesized that firstborns are problem children and that only children are likely to be spoiled due to parental overindulgence (Weiten, 1998, pp. 483-484).
Characteristics of Positions in Birth Order
Studies have shown that environment is not the sole personality determinant. However, environment is not trivial and should be considered, especially because studies regarding birth order have been shown to have some consistency. Firstborns have been shown to be more conscientious, ambitious, academically oriented, conforming, conservative, inclined toward leadership, and respectful of their parents than their later-born siblings. Conversely, children born later in the birth order tend to be more unconventional, flexible, and rebellious (Sulloway, 1997, p. 5). Only children, being firstborn themselves, tend to exhibit traits more similar to those of other firstborn children. However, only children seem to have better self-esteem and are higher achievers than children who have siblings (Brophy, 1989, p. 54).
Children who have siblings must also contend with something that does not affect only children, namely sibling rivalry. Children who have siblings must compete for parental attention and familial resources. Only children do not have to deal with this kind of competition. Not having siblings allows for greater variance of personality types among only children; however, lack of siblings has repercussions for the only child's later social interactions (Koontz, 1989, p. 38).
Stereotypes of Only Children
The only child is automatically stigmatized. When asked to describe personality characteristics of an only child, many people will respond negatively, indicating the presupposition that only children are spoiled brats. In China, couples are encouraged to have only one child in order to help curb population growth. These children, or "little emperors," as they have been called, are generally seen as spoiled monsters. However, research conducted by Falbo (Brophy, 1989), a psychologist known for work in the area of birth order, indicates otherwise. Falbo found that Chinese only children fared no worse in personality or achievement than their counterparts with siblings. However, only children are also often seen as high-achieving, motivated, and successful (Brophy, 1989, p.56).
Because only children lack siblings, they lose the immediate availability of others near their own age with whom to interact socially. In order to develop normal social skills, only children must be exposed to other children of the same age through other means. For example, play groups can be valuable for the learning of social skills. However, only children must work to win friends because family life does not provide them.
Introversion/Extraversion and the Only Child
According to Skinner's behaviorist theory of operant conditioning, only children would undergo conditioning to affect their behavior in social situations. Operant conditioning involves the conditioning of behavior according to the consequences it produces (Mischel, 1993, p. 307). In this way, only children would be conditioned to behave in an outgoing manner, if they are to win friends, because they have no guaranteed familial playmates. Said the pediatrician M. Kappelman, "Only children don't easily assimilate into large groups, and when they do they tend to dominate" (Brophy, 1989, p. 55). This conditioning would take place regardless of a child's natural inclinations toward extraversion or introversion if the child wishes to make friends.
Jung was the first psychologist to describe the inner- and outer-directed types of personality. Inner-directed persons, or introverts, tend to be concerned with the internal world of their thoughts and feelings. Outer-directed persons, or extraverts, tend to be interested in the external world of things and people (Weiten, 1998, p. 483). Because only children have a greater variation of personality types, logic would dictate that introversion and extraversion are equally likely traits in only children.
The psychologist Eysenck (Weiten, 1998), while largely endorsing the role of genetics in determining personality, was also a pioneer in the ideas of extraversion and introversion. He suggested that introverts tend to have higher levels of arousal than extroverts. Therefore, introverts are more easily conditioned than extraverts and, because social situations cause arousal, the heightening of arousal will make introverts uneasy and wont to avoid social interaction. Hence they become introverted (Weiten, 1998, pp. 495-496).
The Dilemma of the Only Child
Because only children do not have siblings with whom to interact, they learn to be children on their own. Parents and play groups can help, but ultimately children become conditioned to depend on themselves. Says one adult only child, "Possibly the best part was developing the ability to enjoy being alone and to entertain myself. I've always had plenty of friends, yet people are surprised by how much of a loner I can be" (Koontz, 1989, p. 39). Although this self-sufficiency can have its benefits, it can also mean that only children are inherently alone as their personalities develop.
Because only children must develop in social situations that may not be suited to their personalities, the concepts of introversion and extraversion must be re-evaluated in the consideration of only children. Ultimately, an only child's environment forces him or her to take on both characteristics of introversion and extraversion despite natural inclinations to be one or the other. A naturally introverted child must show extraverted qualities if he or she wishes to make friends; likewise, a naturally extraverted child must learn to show introverted qualities by being content to focus on his or her own thoughts when playmates are unavailable.
Of course, very few humans are strictly extraverted or introverted; most fall somewhere in between the two. The term "ambivert" has been coined to describe those persons who show both characteristics. However, the term "ambivert" is not accurate in describing only children. To call an only child introverted, extraverted, or ambiverted would be to imply that the child developed into its natural tendency toward that certain personality type with little influence from its environment. Thus only children are caught in a dilemma. Although environmental influence is not the sole influence in personality development, only children must develop their personalities in unique environmental situations. Their environments force them to act against their natural tendencies in order to function normally. These "only-verts" then must always at times be acting in ways against their natural tendencies. Perhaps this struggle helps explain some of the common characteristics that emerge among only children, such as the tendency to not participate in many activities but leading the ones in which they do participate or learning to be comfortable being "loners" by learning to retreat within themselves. Perhaps because the emotional difficulties that only children are prone to have such as excessive sensitivity, hypochondria, or trouble expressing anger (Brophy, 1989, p. 55) are results of environmental influence but not in the way most commonly assumed. Rather than solely the effects of sibling-free socialization, these emotional difficulties could be attributed to an almost Freudian struggle between opposing forces: the natural tendency toward extraversion or introversion versus the environmental pressures to subdue those tendencies in order to function.
Of course, Freud's theory, although testable, cannot be proven or disproven scientifically (Grünbaum, 1986, p. 221) and any situation analogous to Freudian theory would be difficult to test as well. However, a questionnaire could be designed much like one to measure extraversion and introversion with modifications to take into account the special case of only children to try to get some sense of how only children feel about themselves and their interaction with the world around them. For example, a study could be used to determine how adults feel their upbringing as only children affected them.
Ever since Adler brought forth the idea of birth order's effect on personality (and possibly before), the only child has been seen as having distinct personality traits. Although environment has not been shown to be the only influence in personality development, only children develop in a unique social setting. Therefore, perhaps their environments exert enough influence in their development to accentuate personality traits and force a struggle against natural tendencies.
A Monkey Wrench in the Study of Birth Order
As Eischens points out, the only child is a unique breed. Acting as the first- and last-born in a family, the only child takes on a multitude of roles and responsibilities unlike those of any other sibling. The only child breaks down the positions in birth order. The author of the article lays down these differences in a concise manner, taking time to give a strong base to her argument. She then elaborates on the real issue that separates only children from others, introversion versus extroversion.
Although nobody exists as purely introverted or extroverted, the only child cannot show a strong tendency towards one or the other. Instead, both introversion and extroversion become important as the only child is forced to take on both personality types depending on the given situation. Because of the lack of familial bonds, one has to learn to depend upon oneself for thoughts and entertainment. In this isolated environment, the only child takes on the characteristics of an introvert. The lack of familial bonds also becomes important when the child looks for entertainment outside of her- or himself. In social settings, the only child is much more desperate for the friendships of others his or her own age than the child with siblings. In order to make these necessary friendships, the only child must take on the qualities of an extrovert. The result is that the only child is a hybrid of the two personality types. The author does not have empirical evidence of this theory, yet she offers possible tests that would give proof. She does not state her ideas as concrete, but instead offers them as suggestions to understand the differences between the only child and all others.
The shortcomings of the article come in the lack of evidence for the points made by the author. She makes questionable statements as truth and then builds her argument progressively from these. Early in the paper, she states that, "Not having siblings allows for greater variance of personality types among only children." Although this may be true, inadequate evidence is given and the author uses this statement as a base for the rest of the paper. Although it is true that only children have no sibling comparisons for personality development, there are several factors that extend past this difference. It could be argued that a variety of things could outweigh this difference, including genetic aspects and other environmental variables. These things might make the dilemma of the only child minimal.
Also, perhaps sibling rivalry creates personality differences that the only child does not experience. It is true that brothers or sisters will help define personality, but siblings will also be a source of differences. Differences will appear in children because of their desire to define themselves within the family. This goes along with the definition of the word rivalry itself. Instead of emulating other siblings, children may break away in order to win parents' favor. Perhaps only children, instead of emulating siblings, will turn to the other familial constant, and their personality types will be guided more by their parents. It is difficult to take the personality definitions of birth order as truth, and adding the only child as a source of variation is difficult to prove.
Using the variation within the only child, Eischeus goes on to say that, "logic would dictate that introversion and extroversion are equally likely traits in only children." But would not logic also dictate that introversion and extroversion are also both highly likely in all children? All children have to extend outside of their natural tendencies. Home does not provide a complete social outlet, and therefore children are always desperate in social situations. Natural introverts need to interact to build relationships. Likewise, natural extroverts must learn how to function on their own as well. These are things present in all children, not merely in only children. The only child is just a more extreme example of the hybrid nature of personality. The natural tendencies of introversion and extroversion do not disappear in only children, they just become more hazy. Only children establish more of a balance between the two, but can never be rid of the natural tendencies that appear in all children.
Finally, the author does not take the necessary steps to prove that only children do not fit into their stereotypes. Her only evidence comes from a study of Chinese families. Although the study is most likely true, one must question whether the study can be applied to our own culture. Cultural differences could present enormous problems for these findings. The study is from a world that functions differently on most every level, including the family. There are indeed constants in family life the world over, but there are also differences put into place by culture that cannot be ignored. The "high-achieving, motivated, and successful" only children of the study are part of a Chinese community that puts different stress on success and education than our own. It is quite possible that the research is generalizable to all nations, but without another example of such findings, one cannot help being hesitant with the author's proof.
Overall, the author brings to light some very interesting ideas that separate the only child from other children. We are given not only an overview of the importance of birth order, but also a look at a unique role that has qualities greatly different from others. Their social setting leaves definite traces in the personality of only children that prevent them from falling under the characteristics of birth order that have been previously studied. Eischens may leave out empirical evidence to back up her claims, but the claims seem to make sense and it is easy to follow her logic. Based on what she states, the only child brings confusion to the usual stdy of birth order. Her essay finds its success not in proving points, but instead in its ability to make us question and re-evaluate our understanding and study of birth order.
Only Children Develop Unique Socialization Trends; However, Empirical Evidence Is Needed
Purva H. Rawal
"The Dilemma of the Only Child," by Eischens, describes the socialization pressures and effects on only born children. Although the author raises interesting theories, much of the evidence cited lacks an empirical basis. Eischens presents intriguing theories on the introversion and extroversion dichotomy forced onto only children by the inherent fact that, although they have no siblings, they must learn to interact and form meaningful peer relations.
Adler's work is presented in a speculative light, as the author develops her own interpretation for Adler's research and obvious interest in birth order. She establishes well that Adler established that one's birth order is accompanied by distinct characteristics. However, no empirical evidence is cited for his work and following the speculative discussion on his interest in the field, the reader becomes unsure as to whether the discussion on Adler's theory was not also prone to speculation.
The author succinctly and clearly describes the well-established findings of birth order characteristics. The inclusion of descriptions of first-born children and later born children act as a baseline for comparison to only born children through the remainder of the paper. Most people can think back to a time when they were affected by such familial phenomena as sibling rivalry. The effects of such social mechanisms often have important ramifications, both positive and negative, for child development and later sibling relations. Eischens mention the concept of sibling rivalry, but fails to elaborate on a topic that is relevant to many. Sibling rivalry may act as a socializing agent in the sense that it teaches children how to constructively cope with conflict. The comparison of the effects of sibling rivalry to the effects of its absence in only children provides insight into the unique development of only children.
One of the paper's greatest strengths is the inclusion of cross-cultural myths and findings on only children. The relevance and importance of cross-cultural work has often been ignored in psychological research; however, an increasing modern trend toward globalization and cultural understanding has fostered an increasing interest in cross-cultural work throughout the twentieth century. Psychology by its very definition is the study of human culture and it is imperative to construct a culturally holistic picture. Having the advantage of examining other cultures' outlooks on what we may classify as phenomena exclusive to our social realm encourages a more critical and cosmopolitan perception of psychological occurrences.
One of the greatest argued losses to only children is their lack of social interaction within the family structure as a consequence of the absence of potential sibling playmates. A study conducted by Mueller and Vandell (1995) found that children with older siblings offering an outlet for interaction were more responsive socially to other children their own age. This interaction propels them into social situations in which they actively seek out playmates in settings away from home. The foundation for developing healthy peer relations is laid in the home at an early age. This foundation is solidified via the exposure to siblings as they communally partake in activities including social-role playing and scripts. Children are socialized as to acceptable social roles, and actions within those social roles, largely through interaction with their siblings. The author perhaps could have elaborated on this point as it is of utmost importance to the social development of the only child.
The Jungian theory and Eysenck's elaboration on the introversion and extroversion tendencies provide a firm basis for the presentation of the theory that the only child's unique familial environment forces the child to incorporate qualities of both introverts and extroverts into the child's personality. Only children must learn when each trait is appropriate so that they are able to establish healthy peer relations, while also learning to develop on their own in a lonely environment. However, Skinner's theory on operant conditioning, although well laid out, is not tied into the argument of the inclusion of both introvert and extrovert tendencies in only children. Likewise, the few sentences outlining a Freudian interpretation of only child characteristics do not bolster the credibility of the author's argument. Freud's theories lack a scientific foundation and are largely speculative in nature. Nevertheless, the author captures the reader's attention with her theory on the unique personality development of only children.
The paper presents a unique argument for the incorporation of both introvert and extrovert tendencies into the personalities of only children. However, the author is unable to adequately provide support for the theory supplementary to Jung's and Eysenck's contribution. Some of the psychologists cited, such as Freud and Adler, do not add to the author's arguments. Overall, Eischens provides readers with an interesting outlook on the unique personality development and environmental forces involving only children.
The Real Dilemma Lies in the Birth Order Theory
Barbara M. Trzop
Birth order is an interesting, but scientifically weak, phenomenon that attempts to explain how a child manifests the personality characteristics that he or she exhibits. Eischens's paper describes in nice detail the foundations and framework of the theory. However, the lack of empirical evidence does not lend support to a theory that, at best, makes generalized statements about correlative behaviors that may or may not really have anything to do with the order in which children are born.
Eischens's description of procreation seems to be a weak introduction to why birth order is considered important to personality theorists, and to psychologists in general. Of course the goal of human beings is to successfully reproduce themselves, thereby giving rise to future generations. However, what does birth order have to do with this, and why would birth order influence personality, according to this evolutionary point of view?
In order to give this introduction the support it needs, Eischens needed to introduce the theories of sociobiology and, more specifically, evolutionary psychology. As Eischens points out, humans of past generations did not have the advantages of modern medicine that we have today. Infant mortality rates were high, given the vulnerable states of newborns, and the squalid conditions into which many were born. Having a child survive past its first year of life took as much luck as it did resources. Therefore, it made sense that parents would lavish their attention and care on a child who "made it" past his or her critical period. First-borns are guaranteed the most resources, because they are the "survivors." It is better to place one's bets on an older child who has already passed the most critical period of his or her life than to waste them on the weaker, younger child who is not yet guaranteed to survive. Therefore, the stereotypes of the successful, confidant, domineering first-born, and the dependent, overcompensating, last-born could stem from this theory. It is a harsh, almost cruel way of looking at family structure, but it fits the "survival of the fittest" paradigm.
Taking this viewpoint, it makes sense that only children emulate first-borns in their personality structures. Parents who have only one child must make sure that this child survives, because there are no "backups" to take his or her place. Therefore, parents spend all of their resources on the child, molding the child into an individual with the most chance for survival. In fact, the only child should have an even better chance for success, because there are no other children to take resources away from him or her. The child grows up to be an independent, self-sufficient, successful individual, because these are the traits necessary for survival.
However, can one take this viewpoint and say with complete certainty that all only children grow up to be high-achievers? Of course not, because the outcome of any individual depends on the interaction of inherent traits and environment. Any child with high achievement motivation who grows up in a supportive family has the potential for success, whether this child be the tenth-born or the only-born. Of course, a child with personality traits that predispose him or her to dependent behavior may come out another way.
As Eischens points out, only children often have the stereotypes of being "spoiled brats." Eischens does a nice job of explaining the cultural significance of this, by citing examples of a similar phenomenon in Chinese cultures. However, not all "spoiled brats" grow up to be the high-achieving, successful individuals that Eischens makes them out to be. Being "spoiled" often indicates lavish and overindulgent attention by the parents, which sometimes indicates a lack of discipline or control on the parents' part. This type of behavior, interacting with a child's tendency for low acheivement motivation, may result in the opposite effect. The child may grow up expecting things to be done for the child all of the time, and may learn to become dependent on others to do everything for him or her. Living a life of luxury, without the normal, everyday conflicts that having siblings bring, may cause the child to have a distorted view of the world. The transition to adulthood and self-sufficiency might be difficult for such a child, who may find himself or herself seeking others onto whom he or she could cling.
I disagree with Eischens as she says that only children essentially learn to be children on their own. Children will behave like children. This is inherent. An only child does not need to be taught how to play. Also, it is not necessarily correct to say that only children deal with a "unique" environment that forces them to go against their natural tendencies. The natural tendency of a child is to be a child! If the child is introverted, the child will seek out other introverts as part of his or her peer group. The same is true for extroverts. It is no different than any other form of social interaction. Although I agree that sometimes only children may have to learn how to act against their nature (i.e., acting more extroverted in an aggressive play situation), this is not always necessary, as Eischens suggests. In fact, I would go so far as to say that siblings are more prone to act against their natural tendencies toward introversion-extroversion, because they are often forced to play together and to agree with each other. In order to get along, siblings must sometimes modify their personalities, or else they will hear it from their parents! Only children have the luxury of choosing their playmates, and if they find none suitable, they are perfectly happy playing on their own.
Most of these observations come from personal experience, because I myself am an only child. I will be the first to admit that self-report is not always reliable, nor is it always valid. However, I believe that this is the general problem with considering birth order to be a causal factor in shaping personality. It is extremely difficult to come up with a systematic way of doing this. Of course, one could take an inventory of the kinds of traits that children in a specific "birth order slot" possess, and see if there are any correlations. However, this does not seem to be any different than assessing the validity of astrological signs. Do not the signs of the Zodiac dictate traits as a function of birth order? Using this model, one could even argue that people treat one differently according to the month in which one is born. You and I would both agree that this is bad science, with little empirical foundation. I would have to say the same for birth order. Individuals are all different. The interaction of traits and states dictates personality development, and they do not combine in the same way for everyone. Therefore, it is difficult to say that birth order makes a difference. Perhaps birth order makes a slight difference, but I believe that there are more influential causal factors that determine personality in children.
Merely Scratching the Surface of the Birth Order Dilemma
Alissa D. Eischens
In my paper entitled, "The Dilemma of the Only Child," I make an argument for the possibility that birth order can have an effect on the development of personality. I make no claims about the significance of birth order, but I do suggest that it can be a factor in a child's environment that could affect his or her personality. I take the idea of birth order one step further and suggest that perhaps only children are affected in a unique way by growing up in an environment lacking siblings. My suggestion deals specifically with the tendencies toward introversion and extraversion and how only children may have to fight against their natural tendencies. I thank Jones, Rawal, and Trzop for their commentaries concerning my paper.
Jones does well to recognize the ideas I wish to put forth in my paper. In fact, he seems to be so clear as to my intention that I was somewhat taken aback by the criticisms that followed his summary. That said, I wish to address some of the criticism Jones presents.
Jones' main criticism lies in his finding fault with my lack of support for my arguments. I agree that my examination of birth order theory is indeed cursory and warrants further research and empirical evidence to be deemed credible. However, as Jones recognizes when he says, "Her essay finds its success not in proving points, but instead in its ability to make us question and re-evaluate our understanding and study of birth order," he makes my point exactly. I did not presume to build an airtight case for my argument about only children. Rather, I intended to introduce the possibility that birth order can affect only children differently than children with siblings. I offer some evidence as a means to begin building an argument; clearly, I need more support if my ideas are even to be tested, let alone be called a theory.
Jones also makes an argument for the effect of birth order and sibling rivalry on children who have siblings. I agree fully that children who do not have siblings are in a position different than that of only children; it is precisely this difference in situations that I wished to examine while focusing on the only child. Finally, Jones says that families do not provide total social support, and all children, regardless of their birth order, must strive to make friends. I agree and did not mean to argue otherwise; however, I would argue that the presence of siblings at least gives children "practice" in socializing with others their age, whereas only children are immediately "put to the test."
Rawal echoes this sentiment in her commentary. She sites a study in which socialization with older siblings allowed for a sort of social practice that encouraged children to seek out playmates outside the family. Like Jones, she too summarizes my paper clearly. Unlike Jones, she sees the inclusion of information concerning Chinese only children as positively cross-cultural. Jones felt that information to be too selective.
Rawal finds fault with my lack of empirical evidence and my inclusion of Adler's and Freud's theories as support for my argument. I will not repeat my reasons for lack of empirical evidence--I have stated them previously in my response. However, in response to her criticism concerning the inclusion of Adler and Freud, I will comment. She will notice that Adler appears in the introduction of my paper. As one of the earliest psychologists to be interested in birth order theory, I thought it amiss not to include his ideas. As for Freud, I was merely using his ideas as an analogy to help clarify the point I was trying to make concerning the inner and societal struggles with which only children must grapple. I did not intend the inclusion of Freud to add credibility to my argument.
I take exception to the manner in which Trzop offers her criticism. As I have already stated, I intended to introduce the topic of only children and birth order, not support it wholeheartedly or begin to prove it. Although Trzop makes valid criticism, I do not believe making a mockery of my paper is the best way for her to bring her ideas to my or the readers' attention. She first criticizes my introduction. I thank her for her suggestion for the consideration of evolutionary psychology; her suggestion is valid, and should I pursue this topic further it would definitely be taken into account. However, the purpose of my introduction was not to illuminate the harsh realities of survival but rather to introduce the topic of birth so I could move on to point out that people are obviously born in a certain order within a family, and to suggest that perhaps that order plays a role in personality development. Trzop also says, "The natural tendency of a child is to be a child!" and I could not agree with her more. But I would say that although children developmentally go through similar stages, their "natural tendency" when personality is involved cannot be defined. I would ask Trzop to further explain what exactly "being a child" entails. She draws upon personal experience for her argument, and I could do the same if I chose, being something of an introvert myself. But as Trzop recognizes, personal experience is not the best way to prove a point, so I shall refrain from sharing mine. As for her likening birth order theory to being a "bad science" along the lines of astrology, I will not comment. My horoscope told me that avoiding conflict would be in my best interest today.
Brophy, B. (March 6, 1989). It doesn't hurt to be alone. U.S. News and World Report, 106, 54-55.
Grünbaum, A. (1986). Précis of The foundations of psychoanalysis: A philosophical critique.Behavioral and Brain Sciences, 9, 217-284.
Koontz, K. (February 1989). Just me. Health, 21, 38-39.
Mischel, W. (1993). Behavioral conceptions. In W. Mischel, Introduction to personality (pp.295-316). New York: Harcourt Brace.
Mueller, E. C., & Vandell, D. L. (1995). Peer play and friendships during the first two years. In H. C. Foot, A. J. Chapman, & J. R. Smith (Eds.), Friendship and social relations in children (pp. 181-208). New Brunswick, NJ: Transaction.
Sulloway, F. J. (September 1997). Birth order and personality. Harvard Mental Health Letter, 14, 5-7.
Weiten, W. (1998). Personality: Theory, research, and assessment. In Psychology: Themes and variations (pp.472-515). Pacific Grove, CA: Brooks/Cole.