The Contribution of Twin Studies to the Nature-Nurture Debate - Term Paper Example

The Contribution of Twin Studies to the Nature-Nurture Debate Introduction The growth and development of human beings has been a subject of great interest to researchers and scientists for a long time. Many studies have been conducted on this subject with the aim of finding out the processes of character or personality development. From these studies, various theories have emerged emphasizing different aspects of growth and development. These theories can be categorised into different groups such as learning theories, cognitive theories, psychoanalytic theories, psychosexual theories, and moral reasoning theories among others. However, there has been a major debate regarding the influence of nature versus the influence of environment in human development. Apparently, the behaviour of humans is influenced by genetic factors as well as environmental factors. In early research on human development, researchers tended to adopt narrow focuses on either the nature or nurture influence on human behaviour (Collins et al., 2000: 218). Researchers either concentrated on the role of genetic heredity in influencing child development or on how the environment exerted influence on a person. With regard to nature, there is enough evidence suggesting that children inherit some behaviours with their parents’ genes. According to Plomin (1990), the evidence of genetic inheritance in behavior development has been obtained from studies on animal behavior as well as on twins and adoption. Some evidence of genetic inheritance in human development is physical such as skin color, height, sex, and eye color. However, evidence of genetic inheritance can also include inheritance of diseases such as hypertension, leukemia, and dementia among others (see Alzheimer’s Society, 2012). On the other hand, the environment has been found to have influences on human development and behavior. One of the earliest theories suggesting the influence of the environment in behavioral development is the natural selection theory. According to this theory, animal behavior and survival is determined by their fitness in the environment (Orians, 1962). Under the theory of natural selection, the organisms that adapt to their environments have a higher chance of survival than those that are not well adapted (Dingemanse, N. J., & Réale, D. (2005: 1168-1169). The environment plays a major role in the development of traits, survival skills, and behavior in organisms as suggested by this theory. Recent Debates on Growth and Development: Nature versus Nurture Focus has shifted from concentrating on either the genetic or environmental aspects of growth and development. Today, behavioral scientists tend to agree that both genetics and environment influence growth and development. According to Johnson (2007: 423), behavioural scientists are realizing the interplay between genetics and environmental influences in the development of human behaviour. Although there are major differences in the understanding of this interplay, it is now widely accepted that growth and development are influenced by nature and nurture. The nature versus nurture debate is based on the level of influence of inherent or learned characteristics on behaviour (McLeod, 2007). On one hand, the proponents of nature’s role in behavioural development believe that human development, especially physical development, has more to do with genetic inheritance. The proponents of the nature argument consider individual differences are caused by differences in genetic codes. For example, although a couple can give birth to two children, the children will differ in genetic make-up, which may be seen in physical differences such as height, sex, skin colour, eye colour, weight, and vulnerability to diseases. Nature According to the supporters of nature in the debate, the process of maturation results in the emergence of more observable differences. As humans grow from childhood to adulthood, they pass through several stages of development including puberty and adolescence. Each of these stages of development manifests itself in some physical change such as beard growth in men and breast and hip growth in women. These physical changes are not influenced by the environment. For example, it is impossible for a child to have a beard under normal circumstances. This is because beard growth is controlled from within by an individual’s biological clock. Some individuals grow beards earlier than others. Moreover, some people do not grow beards at all, even into adulthood. Some of the major theories supporting nature’s influence include Bowlby’sattachment theory and Chomsky’s language theories. Attachment Theory The attachment theory proposed by John Bowlby is one of the strongest theories supporting nature’s influence in the debate about behavioural development. This theory suggests that children are born with a pre-programmed attachment to a central figure. This central figure is usually the mother. Apparently, children form an attachment with their mothers before they are born. This attachment continues to influence the development of the child, even after birth. Based on extensive studies, Bowlby concluded that maternal attachment was essential for proper development of a child’s mental and cognitive health (VICEDO, 2011: 404). Bowlby’s theory of attachment ignored the role of the environment or external factors. One of his arguments about the role of the culture in psychological development in relation to his maternal attachment theory was that psychological well-being, just like physical well-being, can be independent of culture (Stevenson-Hinde, 2007). This means that the psychological development of a person is not necessarily influenced by external factors such as culture. Based on his attachment theory, Bowlby argues that children who lack maternal attachment, or whose maternal attachment is disrupted, tend to develop psychological and mental problems, which lead to personality disorders. Some of these include domestic violence, dissociative disorders, and addictions (Clearry, 1999:33). Chomsky’s Language Theories Language acquisition is also a controversial topic. Noam Chomsky held the view that language is inert. According to Chomsky, human beings are born with basic language rules. One of Chomsky’s main theories was universal grammar. According to this theory, Chomsky observed that the common patterns and rules used in different languages were not coincidental. Rather, these common patterns and traits indicate that language must be innately produced for it to have such a universal pattern (Baptista, 2012: 352-353; Slabakova, Sumru Özsoy, Nakipolu & Mine 2010: 661). Additionally, Chomsky proposed a theory of innate language. In this theory, he argued that language was too complex to learn or acquire from the environment. Language consists of complex sounds, phrases, and words that would take a lifetime to master. However, he observed that even young children are able to learn language despite their young age. This, according to Chomsky, was an indication that the capacity for language was pre-determined by the human brain (Bergesen, 2004: 367; Putnam, 1994: 330). The environment only influenced language rules. The two theories by Bowlby and Chomsky support the idea that nature plays the leading role in behavioral development. They share the view that the human brain is pre-wired to develop behaviors and personality such as language and psychological characteristics. To a large extent, these theories have valid justifications. They tend to support the view that biological or genetic inheritance primarily determines the development of behavior and physical characteristics. The two theories are supported by Darwin’s natural selection theory. More recent studies in medicine have also supported the principal role of nature. Some diseases are associated with biological or genetic inheritance. Diseases such as cancer, infertility, diabetes, and heart disease have been found to have some genetic links (Alleyne, 2009: Collins, 2011). With the advancement in science and technology, the evidence for the role of nature in influencing development is growing. For example, in the recent past, scientists have been seeking permission to conduct experiments in which children could have three biological parents as opposed to two. These experiments are based on the evidence of genetic influence on child health. Therefore, the experiments are intended to prevent the passing of degenerative diseases and unhealthy conditions from parents to children through genetic inheritance (Barnett & McKie, 2004). Nurture On the opposite side of the nature-nurture debate is the argument that nurture or external factors have a greater influence on human development. According to the proponents of this argument, the human mind is bland at birth. As one grows, the brain is progressively filled with acquired experience from the external environment (McLeod, 2007). The classical empiricists opposed the view of innate ideas and argued that learning occurred through sense (Wren, 2007: 8). The empiricist or environmental arguments showed that the external environment was responsible for the development of human behavior. Some of the main empiricist theorists include John Locke and David Hume. John Locke The principal concept proposed by John Locke was that of tabula rasa. According to this concept, at birth, a baby’s brain is in a blank state (Martin, 2009: 3). The tabula rasa theory proposed by Locke challenged the concept of innate ideas as postulated by supporters of nature’s dominant role. According to this theory, the human brain is continuously filled with new ideas through the interaction of the senses and the environment. The environment acts as the original source of concepts, which are then transmitted to the brain through the senses. The brain then processes the simple cues into complex ideas and stores them. Locke’s theory is supported by the classical learning theories. Classical learning was based on systematic learning processes involving different stages or phases of learning. At the basic level, the learning process focused on simple ideas and concepts. However, the complexity of the learning process increased as the stages of learning went up. A good example of this classical learning approach is the technical-scientific curriculum model. This model uses a systematic approach. It organizes learning into different stages and treats all learners equally. The learning process is considered as universal where learners go through progressive steps. In support of Locke’s concept of learning through senses, let us use an example of a blind person. A person who is born blind has no sense of sight. Therefore, such a person cannot have the experience of color because color because they cannot see. We cannot taste, feel, smell or hear color. However, a person with good sight is able to recognize different colors. This example, supports Locke’s argument that humans learn primarily through experiencing their environment using their five senses. David Hume David Hume supported most of Locke’s ideas. He opposed the argument that ideas were innate. Just like Locke, Hume argued that ideas came from experiences, which originated from the external environment. Hume used the word perceptions to represent contents of the brain. He categorized perceptions into ideas and impressions. Impressions represent sensations, emotions, passions, and desires while ideas include the faint images of impressions in reasoning. Hume argued that people learn through their experiences with the environment. This learning occurs through the senses. Therefore, the more experience one has in the environment, the more knowledge he or she has. With regard to behavior change, Hume’s ideas imply that human behavior is shaped by interaction with the external environment. According to empiricists, behavior is learned. This argument is tested by observing how people change their behavior. There are multiple studies that show the role of environment in influencing risky behavior in adolescents (see Trogdon, Nonnemaker & Pais, 2008: Lovato et al., 2010: Kobus, 2003: DiClemente et al., 2001: Ilesanmi, Osiki & Falaye, 2010). From these studies, adolescent behaviours such as smoking are influenced by their peers, parental involvement, and norms. For example, in a study to examine the relationship between school environment and smoking behaviour in young people, Lovato et al (2010) established that students who attended schools where other students were smoking were more likely to adopt the same behaviour. Moreover, the presence of strict policies to prevent smoking reduced the likelihood of students smoking. This and other studies have provided support to the argument that nurture is a major influence in the development of human behaviour. Twin Studies Twin studies have been conducted to try and resolve the controversy concerning the role of nature versus nurture. In twin studies, scientists observe the physical, behavioral, and cognitive development of twins to try and establish the roles of genetics and environment in their development. Such studies play an important role in behavioral genetics as well as other fields including psychology and biology. In twin studies, researchers rely on data from genetically similar children raised as adoptees or siblings. Primarily, twin studies have been used to compare fraternal/dizygotic and identical/monozygotic twins. The underlying concept in these studies is that if the identical twins develop more similarity than the fraternal twins, this indicates that genes are largely involved in the physical, behavioral, and cognitive development processes than environmental influences. In most twin studies, the findings have supported the role of genes in human development. For example, it is widely accepted that identical twins have greater similarities compared to fraternal twins, adoptees, or siblings (GeneWatch UK, 2009). Twin studies have contributed a lot to the nature-nurture debate. The findings from these studies have either reinforced or challenge some of the positions adopted by either side of the debate. These effects have been in areas of personality traits, IQ, and physical characteristics. Let us focus on each of these areas in more detail. Twin Studies and Personality Traits Personality refers to the range of traits and characteristics that distinguishes one person from another. Various personality theories have been developed to show that different people have different personalities. For example, the big five personality theory identifies five major personality types including extraversion, agreeableness, and openness to experience, emotional stability, and conscientiousness (Leutner, Ahmetoglu, Akhtar & Chamorro-Premuzic, 2014: 59). Apart from the big five personality types, other personality theories suggest different of personality types, including Myers Briggs 16 personality types (Maccoby, 2005; Ellis, 2003; Zarafshani & Rajabi, 2011). The twin studies have contributed to the debate on whether personality traits are influenced by nature or by nurture. Twin studies have reinforced the role of nature in personality traits. First, twin studies have shown that identical twins have more similar personality traits than random people. Identical twins have more genetic resemblance than random people who are not twins. Therefore, the finding that identical twins have more similarity in personality traits suggests that genes have a major influence on personality traits. Additionally, identical twins have more similar personality traits than fraternal twins. Again, this suggests that genes have a large influence on the development of personality. Moreover, biological siblings tend to have greater similarity in personality traits compared to non-biological siblings. All these findings support the view that nature is more influential in the development of personality than environmental factors. However, the twin studies have shown that some personality traits of siblings tend to change as they grow. This has been linked to the influence of the environment. When children are young, they tend to share a lot in terms of environment such as family and home. However, as they grow older, they begin to depart from the shared environments. For example, the siblings may go to different schools where they will meet different peoples and experience different environments. These changes in environment may influence the personality traits of the siblings. Overall, twin studies tend to give more weight to the influence of genes on the development of personality than environmental factors. Although the findings also show that environmental factors are also influential, this influence is limited. Twin Studies and IQ Cognitive development is often related to intelligence. To measure cognitive development, intelligence quotient or IQ is commonly used. Twin studies have also used IQ to evaluate the role of nature versus nurture in cognitive development. In a study of 7,752 pairs of twins including identical and fraternal twins, which was funded by Medical Research Fund, the researchers established that genes played a crucial role in intelligence. The study tested the children’s’ intelligence levels at ages 4 and 14 respectively using various methods including drawing, verbal, and non-verbal tests. One of the main findings from the study was that at age 4, the drawings of identical twins were more similar than those of the fraternal twins. Additionally, the researchers also found similar results in the intelligence tests that were administered when the twins reached the age of 14 years (Kings College London, 2014). The findings from this twin study suggest that intelligence is linked to genetics. This finding is supported by another twin study on genetics and intelligence differences that found a significant effect of genetics on intelligence (Plomin & Dreary, 2014). Despite the significant genetic influence of intelligence, twin studies have also shown that the environment plays some role in intelligence differences. In one such study in the UK, the socioeconomic status of the children was found to account for differences in IQ (Hancombe et al., 2012). In yet another twin study, the findings showed that environmental factors accounted for 40 percent of academic performance in science (Haworth, Dale & Plomin, 2008). The findings from these twin studies support the view that nurture is also influential in the development of intelligence. Although the twin studies find that nature (genes) is more influential in intelligence development, they also find that nurture (environment) influences the differences in intelligence development (see Shakeshaft et al., 2013). These findings contribute to the nature-nurture debate because they support all sides of the debate. Twin Studies and Physical Traits Twin studies have also been used to investigate the role of nature versus nurture in the development of physical traits or characteristics. Physical traits are easily identifiable and may include skin color, height, weight, eye color, hair, and sex among others. Generally, twin studies show that identical twins share more physical features than fraternal twins or non-biologically related siblings. In fact, the word identical is used to refer to the physical resemblance among monozygotic twins. Apart from bodily, physical resemblance, identical twins may have similar habits such as wearing similar clothes, engaging in similar physical activities, and keeping the same hair styles (Plomin, Willerman & Loehlin, 1973: 44). As to whether these physical similarities are genetic or environmental, the findings from twin studies are varied.On one hand, twin studies have shown that genetics are partly responsible for the physical similarities in identical twins. This is especially so with physical attributes such as height, gender, skin color, hair, eye color, and blood type. However, some twin studies have shown that even identical twins can have different physical attributes. For example, some twin studies have shown that identical twins can have the mirror-effect or enantiomorphism. This means that identical twins, although sharing physical attributes, the attributes mirror each other. For example, identical twins can have a habit of sucking different thumbs (Connor, 2013). In yet another twin study conducted to determine the genetic heritability of physical activity or sedentary behavior among a group of identical and fraternal twins, the researchers established that environment factors accounted for over 50 percent of physical activity or sedentary behavior (den Hoed et al., 2013). The findings from the twin studies show that physical traits are determined by nature and nurture. They support the nature-nurture debate. Interplay Between Genes and Environment There is a unique interplay between biology and environment in the development of personality and physical attributes. Although twin studies have shown that genetics account for a major part of development and growth, the role of the environment has also been noticed. While genetic factors remain unchanged, environmental factors change. This affects the development of personality (Thiel & Angleitner, 2010). The changes in environmental factors affect how the genetic factors are manifested. Evidence from twin studies have shown that the changes in environmental factors influence the changes in the manifestation of physical, cognitive, and behavioral factors in human beings. For example, in a longtitudinal twin study spanning 13 years, Thiel and Angleiter (2010) found out that genetic characteristics do not change in twins. However, the variability in twins emerges because of the changes in environmental conditions that the twins are exposed to. This finding implies that twins brought up in different environments may have different personalities although their genetic factors are quite similar. The link between genetic factors and environmental factors is also hormonal response of people to different environmental factors. specifically, when exposed to a perceived threat, even twins will react differenty. The reaction could be to fight or flee. This relationship between the environment and genetics provide evidence that personality development is influenced by nature as well as nurture. Although indirectly, the environment may influence the expression of genetics. The effect of the environment on personality development does not have to be direct. Like in the theory of survival for the fittest, survival means adapting to environmental factors. in a study to determine the interplay between environment and biological factors, Lenroot and Giedd (2011) found out the environment indluences the development of biological factors. Apparently, the changes in environmental factors affect how individuals develop, including twins. Although twin studies have shown that twins living in different environments share a lot in terms of genetics, there is also evidence that the different environmental conditions partly influences the differences among such twins. The differences among twins, including identical twins, suggests that the environment is influential in the development of physical, behavioral, and cognitive features (Hyun and Irwin, 2002). However, the level of environmental influence on human development remains controversial. From different twin studies, the level of genetic or environmental influence varies. However, findings from most twin studies suggest that environmental influence increases with time. As twins grow older, the differences in their personalities or physical attributes become more evident. This is probably as a result of changes in the environment. Since genetics remain stable, the differences in the physical, cognitive, and behavioral attributes among twins can be best explained by the different environmental conditions. In fraternal twins, the differences are more evident than in identical twins. In fact, some fraternal twins may have limited similarities in the way they behave or in appearance (Hyun and Irwin, 2002). However, this does not mean that they do not share some genes. It is partly because of the changes in environmental factors that such differences manifest. Conclusion Twin studies have enhanced the debate on whether nature or nurture is more responsible for the development of personality & physical characteristics. On one side, twin studies have supported the view that genetics are largely responsible for physical, behavioral, and cognitive development. On the other side, the same studies have shown that the environment is also responsible for the development of these traits. While the twin studies indicate that either genetics or nature is more involved in personality development, it also shows that environmental factors influence how these genetic influences are manifested. 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