Executive functions are mental skills needed in everyday life such as planning, reasoning, decision-making, organizing and problem-solving. Working memory, the ability to evaluate information, goal-directed activity and judgment are also executive functions. These skills are controlled by the brain's prefrontal cortex.
Bipolar disorder and other mental illnesses are known to cause problems with executive functioning. Obvious examples are the impaired judgment that can be a symptom of mania or hypomania, and difficulties with making decisions during depression. When reasoning, evaluating and planning are affected by mania or hypomania, the result might be agreeing to chair committees for five organizations even though the time commitment and scheduling would make this impossible. A person in a depressive state may have problems with processing speed, organizing, and problem-solving.
Children and adults with ADHD may have particular problems with executive functions. See What Are Executive Functions? at About.com ADD/ADHD for more in-depth information.