Why 40%?

"40% is that part of our happiness that is in our power to change through how we act and how we think, that portion represents the potential for increased lasting happiness that resides in all of us" (Lyubomirsky, 2007, pg. 6).

What about the other 60%?

50% of the differences among people's happiness levels can be accounted for by their genetically determined set points. This discovery comes from the growing research done with identical and fraternal twins that suggests that each of us is born with a particular happiness set point (Lyubomirsky, 2007, pg. 20).

The most interesting finding is that as the chart shows only about 10% of the variance in our happiness level is explained by differences in life circumstances or situations - that is, whether we are rich or poor, healthy or unhealthy, beautiful or plain, married or divorce, etc. (Diener, E., Horowitz, J., and Emmons, R.A. (1985).