Because of this palpable meshing together of two variables, it is then plausible to establish what aspects of personality are linked to genes, and what aspects most likely exist due to environmental leverage. It has been said that heredity and the environment both contribute 50% to the makeup of an entire human being, but much debate exists about specific percentages and the existence of higher percentages of one factor in different age groups than others (Petrill et al., 2004). Some tactics that have been used in attempts to figure out the many characteristics of human beings include adoption studies, family studies, and twin studies. A review of different topics in human behavior and psychopathology, from human attitudes to the mental disorder schizophrenia, is a beneficial way to broadly explore the argument. Loose conclusions may be derived from these studies, but much more investigation needs to be done, both in creating new research projects and in analyzing previous methodologies and results, before this argument has any potential of being resolved.

Twin Studies

Fraternal twins, or dizygotic twins, share exactly half their genes with each other. They are not as optimal as identical twins for deciphering the degrees of genetic influence, but they are a very good basis for comparison for identical twins. Fraternal twins are similar to first-degree relatives, except they are sure to share the exact same age, as do identical twins. Twin studies usually rely on samples of identical and fraternal twins; if biology has a greater hand than environment, then identical twins should behave or possess psychopathology similar to each other more so than fraternal twins (Plomin et al., 1997). This is an example of the heritability coefficient coming into play: the estimate of how much someone's specific trait in comparison to other people's traits under one characteristic is attributable to genes (Olson, Vernon, Harris, Aitken, & Jang, 2001). This coefficient ought to be higher in identical twins than in fraternal twins. Then again, it is possible for identical twins to express different phenotypes (external expression of genetics) for the same genotypes (genetic makeup). This is representative of their nonshared environments; even though identical twins possess the same genetic makeup, they may go through different experiences throughout their lives that shape their personality, behavior, and psychopathology in ways that make them unique relative to each other (Hughes et al., 2005).

Attitudes

The results of the study showed that differences between attitudes of the participants were at least partially correlated to genetic factors. It also showed that attitudes related to self-reported perspectives or to activities were often correlated. For instance, the survey asked subjects to rate themselves on the trait of sociability. That trait was correlated with 5 out of 6 attitude factors subjects had toward sociability. Attitudes toward athleticism highly correlated with findings on self-reported athletic abilities.

The causal model was expressly supported in these findings, because athletic skill (the mediator), for example, seemed to be linked with attitudes toward athleticism. Of course, this model is not without its problems: one cannot assume that X is the cause of Y in every single situation. Case in point: attitudes toward leadership seemed to be related to high self-ratings of physical attractiveness, sociability, and aggressiveness. Because of these numerous factors, it is still not possible to always accurately assume direct, singular relations between genetic traits and attitudes (Olson et al., 2001).

Interestingly, nonshared environment experiences between pairs of twins seemed to be the strongest cause of attitude variances, overshadowing genetic predispositions as well as shared environment experiences (Olson et al., 2001). Nonshared environment is a term used to refer to something in the environment that directly affects one twin but does not impact the other at all (Van den Oord, Boomsma, & Verhulst, 2000). The study did indicate that some nonshared environment experiences were very much connected to attitudes and self-reports of physical characteristics and intelligence (Olson et al.). This study leads to further questions about nonshared environments: why is it that different environments have so much effect on twins' behaviors and personalities? And why are some attitudes apparently rooted in genetics, while others are not? As previously stated, it is clear that much more research must be conducted on twins before any solid answers can be found.

Theory of Mind

A study was created to investigate this topic, using both identical and fraternal twins. It measured socioeconomic status, verbal ability, and more importantly, the theory of mind of each participant. The first part of the test given to the subjects contained questions that examined their abilities to connect a mistaken belief about a character in stories provided. The second part tested the subjects' abilities to make inferences and their tendencies to attribute a false belief to a belief about characters within the provided stories (Hughes et al., 2005).

Most of the variation between theory of minds of the pairs of twins resulted from nonshared environments. The percentages of influence in decreasing order were attributed to shared environments, verbal abilities, and then genetics. Families with twins are often highly charged with competitiveness, and the more the families discuss conflicts, the more accelerated theory of minds tend to be. This emphasis on environmental influences does outweigh genetic influences on the development of theory of minds in children, but it does not outweigh genes' existence and role entirely (Hughes et al., 2005).

Genotype-Environment Interaction

The study was designed to measure personality traits of extraversion and neuroticism among the twin pairs, traits of impulsivity and monotony avoidance, and family environment and socioeconomic status. Using this information, and taking into consideration that a study like this had never been done before, the researchers came to three different conclusions in regards to genotype-environment interaction. Genotype-environment interaction is a term used by many researchers in relation to twin studies, referring to the potential for people with different genetic makeup to respond differently toward the same external situation (Bergeman et al., 1988). This is an important concept in twin studies because genotype-environment interaction can also be applied to how people with the same genotypes might respond to the same environment.

One particular genotype-environment, labeled Type I, indicates that the environment has more of an impact on individuals with a genotype for low scores on a specific personality trait. Individuals who had low genotypes for extraversion would also score low on extraversion if they perceived their families as high in control or organization, as opposed to individuals who had high genotypes for extraversion. The latter individuals expressed that high extraversion trait regardless of the perceived level of control or organization of their respective families (Bergeman et al., 1988).

Type II genotype-environment interaction was essentially the opposite of Type I. Individuals who have genotypes that cause them to score high on a specific trait were affected by their environments, while individuals with genotypes that caused them to score lower were not affected by their environments. For example, an individual who possesses a high genotype for impulsivity will have that trait increased if she or he lives in a conflict-filled environment (Bergeman et al., 1988).

Finally, Type III genotype-environment interaction was a category only derived from the researchers' study on mice; it was not derived from the Swedish twins. Type III genotype-environment interaction occurs when the environment influences individuals with genotypes that cause them to score higher on traits as well as individuals who have genotypes that cause them to score lower on traits. An environment high in parental control, for example, will restrict the expression of a genotype, while a permissive environment will allow a genotype to emerge as a strong phenotype (Bergeman et al., 1988).

Adoption Studies

Schizophrenia

Another adoption study showed that a high percentage of proband adoptees, or adoptees whose birth parents had schizophrenia, also suffered from chronic schizophrenia or displayed schizophrenic-like behaviors. None of the control adoptees, or adoptees whose biological parents did not suffer from schizophrenia, had schizophrenia themselves, and only a small percentage of them displayed schizophrenic-type symptoms. A current study is also supporting these results, because a significant percentage of proband adoptees displayed some psychotic symptoms, while only a small percentage of control adoptees displayed these types of symptoms. This study also showed that the adoptees whose biological parents suffered from schizophrenia had a higher likelihood of schizophrenia or other related disorders when the adoptive families were low functioning. This speaks volumes for the genotype-environment interaction theory, because of the expression of a genotype being linked to the type of rearing environment (Plomin et al., 1997).

Despite all of this information, it is still very much uncharted territory as to what explicitly causes schizophrenia, and how it may or may not be expressed among adopted children. One of the main difficulties subsists in the deficiency of knowledge on a gene that carries the disorder schizophrenia. It is unknown whether such a gene exists, and doubly unknown to what degree this possible gene influences these types of adoption studies (Loehlin, Willerman, & Horn, 1988).

Infant Shyness

Adoptive parents were given questionnaires that asked them to rate their infants' shyness levels, and then to rate themselves on the traits listed previously. It must be noted that the self-reported ratings of the biological were performed before the birth of the infants, and the scoring of the infants' shyness were performed by the adoptive parents when the babies were two years old. The results showed that in nonadoptive families, the parents who reported high rates of shyness, low rates of sociability, and high rates of introversion also had shy infants. This was also seen in adoptive families whose parents rated similarly, indicating that a combination of home environment and genetics must come into play. One significant conclusion was made in this study that was based on the fact that biological mothers rated high in shyness, and their adopted-away babies were also shy. This strengthens the possibility of a genetic link overshadowing family environment, but of course further research must be done (Daniels & Plomin, 1985).

Children's Adjustment to Divorce

Measures of the study included age of the probands at the time of separation and/ or divorce, self concept (self-esteem) of the probands, social ability, academic ability, behavioral and/or emotional problems, loneliness, and substance use. Of course, the type of adjustment processes that children from biological families went through could be attributed to biology or environment, while adjustment for probands would have had to be linked to environmental processes. The results showed that probands' adjustment to divorce in terms of social ability, self-concept, and academic accomplishments were at least partially genetically influenced, but that their psychopathology could be attributed to environmental factors (O'Connor et al., 2000).

Antisocial Personality Disorder

It was also found that adoptees experienced an even higher risk for antisocial personality disorder if both their biological parents and their adoptive parents came from criminal backgrounds. However, methodological problems exist with these kinds of studies because there are so many factors to consider. For example, it has yet to be clarified whether this disorder is more likely to be carried through the biological mother, or the biological father. Most of these adoption studies were conducted using only information from the biological mother, and not the other half of the equation: the biological father. Information is also vague regarding a criminal background as an instant checkmark for antisocial personality disorder in biological and adoptive parents. It is often assumed that the existence of a biological parent's criminal background immediately means that that parent has antisocial personality disorder, and also has definitely passed it down to the adopted-away offspring. The problem is, it also cannot be assumed that the lack of a criminal background points to a lack of the disorder itself (Roth & Finley, 1998).

Interpreting the results of adoption studies is very difficult for the aforementioned reasons, and it is also challenging to make valid conclusions due to the fact that adoptees already display a higher rate of antisocial personality disorder as compared to the general population. Ironically, the adoptive family environment is often better in terms of care, education, stability, and health in comparison to families in the rest of the population. Adopted-away children, however, are often placed in adoptive family environments similar to their original, biological family environments. Genetic factors are thus "simulated" when the adoptive family environment is similar to the biological environment (Rhee & Waldman, 2002). With all of these discrepancies and uncertainties, it is undoubtedly a complex process to try to figure out what factor has the most effect on the development of antisocial personality disorder.

Family Studies

These kinds of studies are most often used to determine the risk of passing down mental disorders to offspring within families. It must also be taken into consideration that these types of studies do not tangibly express outside factors, such as family environment and culture. These studies are performed using molecular genetic studies, where DNA is extracted from participants' blood samples and the correlation between the DNA and the observed behavior is projected. The most common molecular genetic study is called linkage analysis. This type of study tries to locate a specific gene on a chromosome in the human body. If a gene for a particular mental illness is being searched for, researchers identify an already-recognized gene on the chromosome and label that as a marker. That marker's location, and the location of the actual diseased gene, is very important: the closer the two are, the higher the likelihood that the disease and marker genes will be passed on together, or linked together (Jang, 2005).

Bipolar Disorder and Schizophrenia

Studies carried out on bipolar disorder showed that first-degree relatives of people with bipolar disorder suffered a higher risk for some related mental disorders including bipolar I disorder, bipolar II disorder, schizoaffective disorder, and recurrent unipolar disorder. However, there was not an increased risk for schizophrenia itself (Berrettini, 2000).

Studies carried out using first-degree relatives of people who suffered from schizophrenia showed similar results. Those relatives were at higher risk for schizophrenia, schizoaffective disorder, and recurrent unipolar disorder, but not for bipolar disorder. Interestingly enough, first-degree relatives of people who suffered from both bipolar disorder and schizophrenia did experience higher risk for schizoaffective and recurrent unipolar disorders. This indicates an overlap and suggests a possible partial overlap in familial risk for bipolar disorder and schizophrenia (Berrettini, 2000). A larger-scaled study needs to be conducted before this can be fully determined.

Suicide

First-degree relatives of suicide victims, also called suicide probands, had a greater likelihood of suicide attempts, but it also has to be taken into consideration that they also had an increased risk of other psychological disorders. The suicide probands who had high rates of aggression also had higher risk for attempts at suicide. The increased risk for suicidal behavior in suicide probands, it was concluded from this study, is probably a trait independent of Axis I and II psychiatric disorders (Brent et al., 1996).

Eating Disorders

This study's results showed that eating disorder probands experienced a higher risk of eating disorders, major depressive disorder, and obsessive-compulsive disorder. Substance abuse disorder was placed at higher risk for bulimia probands than for anorexia probands, and obsessive-compulsive disorder was placed at higher risk for anorexia probands than bulimia probands (Lilenfield et al., 1998).

It was concluded that there was definitely a link between people who suffered from eating disorders and their relatives' risk for suffering from the same problems. A plausible connection between the probands' risk for major depressive disorder, substance abuse disorder, and obsessive-compulsive disorder, however, could not be proven. The only theory that could be somewhat supported was that the traits for obsessive-compulsive disorder could cause a familial risk for anorexia (Lilenfield et al., 1998).