Siblings share both genes and environment and therefore the same nature and nurture factors, but yet siblings are often so different. In theory, siblings sharing the same nature and nurture conditions should be incredibly similar. However this is not the case, as we are well aware by thinking about how opposite you may be to your own siblings. This challenges both views, genetic (nature) and environmental (nurture).

Twin studies
Research of monozygotic (MZ) and dizygotic (DZ) twins gives evidence supporting the nature argument–MZ twins share a great deal more similarities than DZ twins do. MZ twins are genetically identical; DZ twins, in the same way as regular siblings, share 50% of the same genetic material. However, twins also share the same nurturing environment from the point of fertilization, so surely nurture can’t be what accounts for the difference between twins?

Twins reared apart
If the similarities between MZ twins was to be as a result of nurture, then it would be expected that MZ twins reared apart would be very different. Studies of twins reared apart however, have shown just the opposite and give much support to the idea that the similarities in twins is as a result of nature. The Minnesota twin study concluded that, ‘on multiple measures of personality and temperament, occupational and leisure-time interests, and social attitudes, MZ twins reared apart are about as similar as MZ twins reared together’. Other findings also support this conclusion; Bouchard and McGue (1981) found that MZ twins reared apart had a mean correlation of 0.72 (compared to 0.86 for reared together). Siblings reared together were 0.47 compared with 0.24 for reared apart–showing some environmental component. These studies although supporting that the majority of similarities between MZ twins is thanks to nature, they fail to explain the proportion not accounted for by nature. Studies of adopted children also draw the same/similar conclusions about nature’s influence of these factors.

Unshared environments
Although siblings can be said to share identical environments (i.e. same home, same family etc), this isn’t really accurate. Individuals in the same home have different experiences which creates different environments. So this could explain for the differences between twins not accounted for by nature. The different environments which individuals have is called the ‘unshared environment’, and can be influenced by age, gender, peer influences, hobbies etc. Furthermore, there are chance factors such as illness and accidents, that shape each of us–these are forces of unshared nurture. So if this is so important, why is it so often overlooked? Research has shown that our own natural characteristics will create our own unshared, microenvironment which is related to these innate characteristics. This means, that it all comes back around to the influence of nature. Your innate characteristics shape your environment, which in return shapes your development. This leads us to believe that nature and nurture are actually inseparable.

Nature and nurture as inseparable
The classic example of this is that of phenylketonuria, an inherited metabolic disorder preventing an individual with this disorder to be unable to metabolise phenylalanine, resulting in a buildup of poisonous substances that cause brain damage. If this illness is detected at birth, the individual can alter their environment, to have a diet which avoids phenylalanine and thus prevents the potential brain damage. Nature (phenylketonuria) is not expressed because of an altered environment. Blakemore and Cooper’s 1970 study of perceptual development in kittens, also shows evidence that nurture (the environment in which you live) can have a profound effect on the development of innate systems.

So, why are siblings so different if they share the same genes and similar environments?
It is clear that the differences in siblings which are not accounted for by genetics are as a result of our unshared environments, while sibling similarities are largely, if not completely, as a result of nature. The unshared environment deserves our attention, it is the environment that is unique to each individual and leads us to explore nature and nurture in a different way. We should also be aware of the extent to which nature creates its own nurture.