Jack was raised as a Jew by his father. Oskar was raised by his mother in Germany. So when the pair were reunited as part of the study they discovered that they shared similar temperaments and mannerisms. Furthermore, they shared certain idiosyncrasies: both liked spicy foods and sweet liqueurs, were absentminded, flushed the toilet before using it, and liked to dip buttered toast in their coffee (Atkinson, Smith & Hilgard, 1987).
This study does provide evidence for a biological basis to personality. It eliminated the argument that twins being raised in similar environments would therefore have similar personalities due to environmental factors. It focused solely on biological factors, due to the fact that the twins were raised in dramatically differing environments. The study showed that one's biological base seems to be more stable than the influencing environment, and thus provides us with credible evidence. .
Lets now turn our attention to the evidence that frontal lobe brain damage provides to us. Arguably the most famous case of severe brain damage is that of Phineas Gage. A construction worker, Gage was involved in a horrific accident, in which an accidental explosion of a charge he had set blew his tamping iron through his head. The tamping iron went in point first under his left cheekbone and completely out through the top of his head, landing about 25 to 30 yards behind him. Amazingly, he survived, but unfortunately most of the front part of the left side of his brain was destroyed. This meant that the functioning of his frontal lobe would be severely affected, if not abolished completely.
Months after the incident occurred, Phineas felt he was strong enough to resume work. But because his personality had undergone dramatic changes, the contractors who had previously employed him were not willing to give him his place again. He had been their most capable and efficient foreman before the accident occurred.