Despite recent intense interest, happiness studies have been impeded by some conceptual and methodological problems, including viewing happiness (well-being/welfare) as different over different persons, as relative, multi-dimensional, non-cardinally measurable, interpersonally non-comparable and using non-cardinal and interpersonally non-comparable methods of happiness measurement. Using the evolutionary biology of happiness, this paper argues that happiness is absolute, universal, and uni-dimensional and is also cardinally measurable and interpersonally comparable. This is needed to make choices motivated by reward (pleasure) and punishment (pain) consistent with fitness maximization. However, happiness indices obtained by virtually all existing methods of happiness measurement are largely non-cardinal and non-comparable, making the use of averaging in group happiness indices of dubious philosophical validity. A method of measuring happiness to give cardinal and interpersonally comparable indices is discussed. These may contribute towards the more scientific study of happiness that is based on sounder methodological grounds as well as yielding more useful results.