What determines an individual's temperament?

Temperament is determined in large part by our genes, and patterns appear early in life (Rutter, 2010). Identical twins reared in different homes often have similar temperaments (Casalin, Luyten, Vliegen, & Meurs, 2012). For example, some infants tend to react positively to new experiences in life (Rutter, 2011), such as the willingness to try new foods and adjust to change, whereas others are more irritable and "difficult" and react negatively to new situations. Another example, is Abang Dan and Kak Nel going to tadika. Abang Dan tak nangis, while Kak Nel nangis nak mak.