Personality: Nature vs. Nurture or something in between?

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Biology 202

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Personality: Nature vs. Nurture or something in between?

Tamara Tomasic

Everyone has a unique personality, a way of experiencing the world and participating in it that is all their own. But is it really? Are our personalities truly unique, or are there others (perhaps thousands of others) with the same personalities as ours? And how is personality determined? Many ways have been proposed in different periods in history and even today. These different propositions can be placed into three categories: personality is caused/influenced by the environment; personality is inherent in our genetic make-up; personality is a mix of both genetic and environmental influences.

In earliest times the most common explanations proposed for personality were dependant on environmental/exogenous factors, such as the day of one's birth or the "demons" that surrounded us in this world. Today still there are those who firmly believe in things such as astrology and that the time and day of your birth will determine your personality. The Greek zodiac is still in use, and (more seriously) Chinese astrology.

In Chinese astrology, entire birth charts are organized in order to determine which signs will hold their influence over an individual in the coming year, and elemental theory is employed to do the same ((1)). In the Greek zodiac birth charts are used as well, but the continued influence of the stars does not change throughout one's life: it is determined by their arrangement at the date and time of one's birth. Numerous websites and books exist where one can look up a "unique" horoscope for the day that one was born; however, these "unique" horoscopes are the same for all those who happen to be born on the same day, sometimes even at the same time. These two astrological theories produce blanket personalities, a one-size-fits-all approach to personality determination.

If personality were determined solely by environmental factors, then it makes sense to think of personalities are only coming in a specified number and applying to more than one person. However, upon closer examination of people it is found that personalities are not the same, even for people born on the same day (example: twins) in the same circumstances, thus disproving the blanket theory that environment is the sole determinant of personality. Another point in favor of a non-environmental approach are siblings and the differences in their personalities. Born to the same parents and raised in the same environment, siblings will still exhibit very unique personalities, different not only from each other but from their parents as well.

Another theory is one of genetics: that personality is passed down from parent to child, or that persons who exhibit similar characteristics will exhibit similar personalities. This theory is the basis behind Japanese thought that personality can be determined by one's blood type ((2)). This process, called "blood-typing" ((3)), is sometimes still used in Japan today for hiring practices and divisions of work in a company. In surveys of Japanese college students, 48% said "blood types make personality 'extremely different' or 'considerably different'" while 96% in another study said "blood types 'have much to do with' or 'have something to do with' personality" ((3)).

Another example of the genetic theory is that of inherited personality disorders, which constitute a type of personality in themselves. If personality disorders are inheritable, why not personality in general, or at least a higher predisposition to a personality? If depression and borderline personalities/predispositions can be inherited, why not a predisposition to a cheerful and outgoing personality? In this argument we would see identical twins with the same personalities, or very similar ones resulting from identical predispositions. This, however, is not generally the case, and there are always some variations that cannot be accounted for by genetics alone, as well as some similarities that cannot be accounted for by random acts of environmental effects.

This is where the third category comes in, a mixture of both genetic and environmental factors. If personality predisposition can be inherited, as is shown in the inheritance pattern of personality disorders ((4)), then ordered personality might be inheritable as well. The inheritance piece itself, however, is not enough. Environmental stressors/influences are what causes the predisposition to develop into a disorder/personality. Here is a study that was done with ASPD (antisocial personality disorder) inheritability in adopted children whose biological and adoptive fathers either did or did not have ASPD ((4)). The study showed:
24% of sons developed ASPD if both biological and adoptive father had ASPD
20% developed ASPD if biological father had it and adoptive father didn't
14% developed ASPD if biological father didn't have it and adoptive father did
13% developed ASPD if neither biological nor adoptive father had it
This study is a great example of how inheritance itself is not enough (the difference between those not predisposed but raised in an ASPD environment and those not predisposed and not raised in an ASPD environment is insignificant), nor is environment the only cause of the disorder (there is not a significant difference in the numbers for the sons who were "predisposed" but raised in a non ASPD environment and those who were predisposed and raised in an ASPD environment). We could then conclude that both the genetic predisposition and the environment play a role in the development of the disorder, and apply this theory to personality. This accounts also for the differences in personality of genetically identical twins, and siblings raised in the same environment born of the same parents.

Personality is difficult to determine, especially because there is not concrete place where it is stored or created; no one can really find the "personality center" of a living being. But the evidence collected from observation and careful study can lead us to believe that there is no one method by which personality acquisition and differences can be explained; only by a mix of internal and external factors, predisposed and random can personality be formed.


1)Year 2000 of Chinese Golden Dragon Year

2)Personality Traits by Blood Type: A Japanese Perspective

3)Special Topic: "Blood Typing" is Still Popular in Japan

4) Butcher, James N., Jill M. Hooley, Susan Mineka. Abnormal Psychology, 12th edition. Pearson: Boston, 1994. 372-376.

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