Is There More To Why Siblings Differ?

August 24, 2008

It is a fact well-known that siblings – despite being closely related – are not copies of one another, neither in appearance nor personality. The reason for this has been the subject of debate and reasonable explainations have been offered. Although the question may be considered answered, I cannot help but supplement such an answer with a theory of my own.

If one is presented to a group of strangers and is told that two of them are siblings, one can rather easily pick them out because they are more similar to each other than they are to the unrelated strangers. Despite their likeness, however, siblings differ; they may share the shape of their nose but not the colour of their eyes – or the other way around.

The reason for why siblings are alike in appearance is because they share 100 % of their genetic heritage – as they have the same parents – but they differ from one because they share only 50 % of their genome. The 50 % unique to each sibling is what accounts or their differences.

Personalities are however not the same as appearances, and if one is presented to the ideas of a group of strangers, it is very hard to decide which two ideas have been developed by a pair of siblings.

The question of why this is so – why the personalities of siblings differ – has been answered by several people, though two may be mentioned in particular for having offered satisfactory solutions to the problem.

One of the two people, whom I am referring to, is Frank J. Sulloway. He argues that differences between siblings are to accredited to their birth order, which offers them different familial niches to occupy: the firstborns face a world of limited competition and thus grow up to relate to authority figures such as their parents, while younger siblings become rebels because their benefits do not come as easily.

As the eldest in a sibling group of five, I see the potential of Sulloway’s theory as many of the traits assigned to firstborns (conscientousness, social dominance and limited agreeableness) also are present in my own personality (though I myself would like to claim that the latter is not the case ;D). However, for the same reason – being one of many – I also realise that his theory is not perfect as not all of my younger siblings are rebels.

So, what about the other explaination offered?

The psychologist Judith Rich Harris advocates a theory presented in her book The Nurture Assumption, claiming that the parental influence matters little when a child’s personality is formed; that the similarities that exist between parents and their children instead are to be attributed to their shared genes. Further, she argues against the influence of birth order, and instead claims that it is a child’s peer group which influences their personality; that children modify their behaviour to fit into the group, something which ultimately has an effect on their character.

As with Sulloway’s theory, I realise that there is truth also to Harris’s, as the environment which is shared between siblings (the family) may be disregarded from as accounting for differences because it is the same for all and thus cannot influence individual differences. Further, I personally am somewhat of a “cultural chameleon”, easily modifying my behaviour to what my current group expects of me.

Because I know myself, I also realise that Harris’s theory is flawed, as I believe little of my own personality has been influenced by what my peer group(s) expect of me. This may of course be attributed to the fact that I am most headstrong and usually disregard from what other people expect of me – though I act according to their expectations I disregard from them in all other ways. Also, as most children’s peer groups change over the course of their lives, it may be interpreted as that their personalities change accordingly. However, this is not the case, as a child’s way of being changes little past their 7th to 10th birthday, depending on the child.

As there is truth to both the theories presented above, they can not be considered as wrong or faulty, but they are indeed flawed, having failed to fully explain the reason to why siblings differ. Thus, I theorise that there is more to the answer of the question, and that the missing aspects are present in the genes.

The ultimate goal of all life is reproduction and such a fact must also be present in the reason for why siblings differ; the differences must in some way increase the survival of the associated genes. I believe this indeed is the case; that siblings differ because their differences ensure the survival of their shared genes.

Allow me to elaborate:

If differences in personality are determined to the majority by genes and to the minority by external influences, then this fact also accounts for why there also are similarities between the characters of siblings, as they share 50 % of their genes.

Differences in personality mean that the siblings will prefer different items and aspects. Not only does this increase the availiable amount of resources – as one child may love beets and the other corn – but it also ensures increased genetic variation because different genes determining a person’s character prefer to mix with different sets of genes present in a mate; crudely, one sibling may prefer mates with brown hair while the other sibling prefers red hair.

The result of this is that the genes shared between siblings may mix with many different sets of genes, increasing the variation. If one person reproduces with many mates, its offspring’s genes will be variated; one version per child. However, imagine if a couple of people with a great number of shared genes – like siblings – reproduce with many mates which differ greatly between one another – because of the different siblings’ differing preferences, – then the resulting offspring will be more variated than a single person’s (or similar sibling-preferences’) ever could be.

As genetic variation is the key to evolutionary success, the genes which make the personalities of siblings different will be more successful in the population than the genes which give siblings similar personalities, and such genes thus spread to eventually become universal.

To conclude and summarise my reasoning:

The personalities of siblings differ because their resulting preferences then will also will differ; allowing the siblings’ shared genes to attain greater variation within the resulting offspring, as each sibling will consider different sets of genes the most attractive. Thus, genetic variation is increased, rendering some of the offspring more resilient than the rest; giving the shared genes a chance of success otherwise impossible had the offspring’s genes been too similar. Subsequently, the genes influencing personality differences between siblings will become more successful and eventually spread over the whole population, as they never are to encounter an evolutionary dead-end.


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