Given that people with borderline personality disorder are impulsive, we would expect them to do poorly on tests that measure functioning of the____.

Psychology question here.

Personality disorders (PD) are a group of psychiatric conditions characterized by experience and behavior patterns that cause serious problems with respect to any two of the following: thinking, mood, personal relations, and the control of impulses.
The functioning of the left side of the brain is associated with analysis and thinking, the right side with emotion, feeling, and impulse.

http://www.1000ventures.com/business_guide/crosscuttings/mind_how-it-works.html

Without a specific context, from the sentence you give, it is hard to tell what you are searching for. If you are speaking in psychoanalytic terms, it might be "superego." If you are talking about brain physiology, it could be something like "frontal lobe."

I searched Google under the key words "borderline personality disorder" to get these possible sources:

http://www.nimh.nih.gov/publicat/bpd.cfm
http://www.palace.net/~llama/psych/bpd.html
http://en.wikipedia.org/wiki/Borderline_personality_disorder
http://www.stanford.edu/~corelli/borderline.html
http://www.psycom.net/depression.central.borderline.html

I hope this helps a little more. Thanks for asking.

Given that people with borderline personality disorder are impulsive, we would expect them to do poorly on tests that measure functioning of the frontal lobe.

The frontal lobe of the brain is responsible for executive functions, such as decision-making, impulse control, and planning. Impulsivity is a common symptom of borderline personality disorder, which means that individuals with this disorder may have difficulty controlling their impulses and making thoughtful, calculated decisions. As a result, they may struggle on tests that require logical reasoning, inhibition of impulsive actions, and problem-solving skills, which are all functions attributed to the frontal lobe.

To arrive at this answer, I combined the knowledge of the symptoms of borderline personality disorder, which include impulsivity, with an understanding of the brain's frontal lobe and its functions in executive decision-making.

Given that people with borderline personality disorder are impulsive, we would expect them to do poorly on tests that measure functioning of the prefrontal cortex. The prefrontal cortex is responsible for executive functions such as planning, decision-making, and impulse control. Impulsivity is often a symptom of dysfunction in this area of the brain.