NUMERICAL ADVANTAGE. “Selfing can evolve in response to an extremely diverse array of selective agents, but its intrinsic 3 : 2 transmission advantage is ever present (up to 50%). This specific advantage arises because a selfing individual, in a stable population, transmits a total of three copies of alleles: two through self-fertilized seeds, and a single copy as an outcrossed pollen parent.”
http://onlinelibrary.wiley.com/doi/10.1111/nph.12182/full