Grühn, D., Kotter-Grühn, D., & Röcke, C. (2010). Discrete affects across the adult lifespan: Evidence for multidimensionality and multi-directionality of affective experience in young, middle-aged, and older adults. Journal of Research in Personality, 44(4), 492-500. doi:10.1016/j.jrp.2010.06.003.