It has been known, probably since humans were human, that many personality traits are hard-wired in the genes and that much of development consists of a gradual revealing of those traits in the same way that physical characteristics are gradually revealed. Thanks to the miracle of the human cerebral cortex, some learning and adaptation occurs but the hard-wired things remain present even if hidden by adaptive maneuvers (eg covering up shyness or hypersensitivity).
Long before you could express yourself with words, you were giving away the signs of your adult temperament. Christian Jarrett explains how.