Phineas Gage

The prefrontal cortex is the last to develop; it is significantly proportionally larger in humans when compared to other species and relates to intelligence, self-awareness, consciousness of perception & experience.  It is important in the various traits that separate humans from other species; namely our awareness of our existence and executive functions including:  Motivation, decision-making, responsibility, social control, rule learning, “willpower”. It is interconnected with brain regions involved with emotion (amygdala) arousal systems, and attention 

There may be reduced volume in individuals with psychological disorders such as depression, sociopaths, schizophrenics, ADHD.  This is why many sociopaths can commit heinous acts without feeling much guilt or sense of social injustice.   

Phineas Gage (1823-1860) was a friendly man who had respect of his peers and coworkers as a man of integrity and fun to be around.  Following a tragic railway accident, a metal rod pierced his maxilla, orbit, and prefrontal cortex in the fashion of a javelin.  He survived the accident, but was subject to peculiar changes in personality.

Pre-accident Gage as hard-working, responsible, and "a great favorite" with the men in his charge, his employers having regarded him as "the most efficient and capable foreman in their employ."  After the accident Gage was described as
.... fitful, irreverent, indulging at times in the grossest profanity (which was not previously his custom), manifesting but little deference for his fellows, impatient of restraint or advice when it conflicts with his desires, at times pertinaciously obstinate, yet capricious and vacillating, devising many plans of future operations, which are no sooner arranged than they are abandoned in turn for others appearing more feasible. A child in his intellectual capacity and manifestations, he has the animal passions of a strong man... 

This case lead to a great curiosity about the functions of various parts of the brain, and remains a relevant story about the importance of the pre-frontal cortex.  It generates a curiosity about how much of one's personality or uniqueness is limited to a particular part of the brain and if the uniqueness that make humans a distinct species would be adapted or shuttled to a different part of the brain in the case of such a lesion.  Also, in the midst of growing concerns over brain trauma due to contact sports such as mixed martial arts and football, how much of that goes on to affect a person's daily activities and personality.  More on that later.

http://en.wikipedia.org/wiki/Phineas_Gage