The Panel Study on Income Dynamics is used to study the trends in the elasticity of son's income in relation to parental income. The study reveals a nonlinear trend that increased for sons born between 1949 and 1953 and then declined for sons born after 1953. The direction of this linear trend is dependent on the time period and could be upward, downward, or static. This trends and other factors taken together explain the different estimates for mobility.