“Note that the 40 percent allocation to intentional activity is a value obtained by the method of subtraction. If we assume that 50 percent of all variation in happiness is genetic, that 10 percent is circumstantial, that intentional action is the only remaining source of variation, and that the estimates are free of error, then the remaining 40 percent must be attributable to intentional action. If these assumptions are met, the method of subtraction has logical force. But are they met?”
“Suppose you learned that 50% of the variation in body height was due to genes and 10% was due to circumstance (e.g., diet). Would you conclude that 40% is due to intentional action?”