Just as there's the 80-20 rule in business (80% of your company's work is done by 20% of its people), there's a 90-10 rule in psychology. In the How of Happiness, Sonja Lyubomirsky shows that only 10% of your happiness is determined by external factors like money, while as 90% is determined by internal factors like temperament and intentional happiness activities.

Similarly, what I noticed in my cognitive therapy sessions, is that 90% of my negative thoughts were all in my head, and could be easily disputed and moved past. In the other 10% of cases, the disputations led to insight and then action items, such as choosing new directions for my career, and taking a new tact with a relationship.

Where else in psychology could this 90-10 rule apply?