Aperiodicity as a Factor in Choice.

@article{Herrnstein1964AperiodicityAA,
  title={Aperiodicity as a Factor in Choice.},
  author={Richard J. Herrnstein},
  journal={Journal of the experimental analysis of behavior},
  year={1964},
  volume={7},
  pages={179-82}
}
Four pigeons were trained to peck at either of two response-keys. Pecks at either key occasionally produced a secondary reinforcer, in the presence of which further pecks occasionally produced food, the primary reinforcer. All pigeons showed a consistent preference for variable (as compared to fixed) interval schedules of primary reinforcement.