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On the psychology of freemium:

- Why do teens spend real money in virtual worlds? A consumption values and developmental psychology perspective on virtual consumption
  http://www.sciencedirect.com/science/article/pii/S0268401214001030

- What Is a Variable-Ratio Schedule?
  http://psychology.about.com/od/vindex/g/def_variablerat.htm

- The Psychology of “Freemium”
  http://www.psychguides.com/interact/the-psychology-of-freemium/

On the popularity of freemium:

- An Empirical Study of the Freemium Strategy for Mobile Apps: Evidence from the Google Play Market
  http://aisel.aisnet.org/icis2012/proceedings/EconomicsValue/14/