Charlesworth and Charlesworth (1981) applied similar logic to both outcrossing and selfing species, and created a model that predicted when dioecy would be favored over hermaphroditism, and vice versa. The model predicted that dioecy evolves if investing in one sexual function has accelerating fitness benefits than investing in both sexual functions, while hermaphroditism evolves if investing in one sexual function had decreasingly lower fitness benefits. It has been difficult to measure exactly how much fitness individual plants are able to gain from investing in one or both sexual functions, and further empirical research is needed to support this model.