Unless people are identical twins, their genetic make-ups are slightly different; hence their organisms, including their brains, are somewhat different (even identical twins’ are, in fact). To thrive with the quirks of their organisms and brains, they often have to behave, think and feel differently. Hence, their personalities are slightly different.
The more people differ genetically, the more unique their organisms and brains are, and the more likely they are to rely on different patterns of behavior, thinking, and feeling to find the snuggest possible fits with the circumstances available to them.
It is not uncommon to assume this statistical trend to mean that there must be specific genes for certain traits, making us somehow hardwired for our traits. And of course many find this conclusion controversial.
But there is no evidence for any of this. For example, although people’s different extraversion levels are partly due to their genetic differences, we know of no genes specifically responsible for extraversion—or any other personality trait, for that matter. Likewise, it is common for people’s personality trait levels to change, either spontaneously, as a result of their experiences, or even because they deliberately want to change.
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As a result, there is no direct link between genes and traits. Any trait level can correspond to innumerable gene combinations and the other way around: The same genetic background can lead to different levels of many traits.
