Effort is slow but steady in the case of a variable-ratio schedule and personal motivation is as important as the anticipation of a reward in maintaining persistence and effort. An example is waiting for a letter in the mail. There is little you can do to make the letter come faster, but you can check the mail box each day. To account for possible changes in personality and for the relationship of personality change to life experience, psychologists have theories of personality development. They use attitude and personality tests to document the change or retention of traits between adolescence and early adulthood.
Most people retain their views unless they have experienced a major life-altering event.
Psychologist Daniel Levinson developed a theory which outlines the developmental stages experienced by adult males. The decremental model of aging supports the notion that mental and physical decline is a natural part of the aging process. This is a narrow view of the process of aging and leads to ageism, or discrimination based on age.