Human self-control research is typically modeled by using a token economy system. A token economy system is a behavioral program in which individuals in a group can earn tokens for a variety of desirable behaviors and can cash in the tokens for various backup, positive reinforcers. The difference in research methodologies with humans - using tokens or conditioned reinforcers versus non-humans using sub-primary forces suggested procedural artifacts as a possible suspect. One aspect of these procedural differences was the delay to the exchange period. Non-human subjects can and most likely would access their reinforcement immediately. The human subjects had to wait for an "exchange period" in which they could exchange their tokens for money, usually at the end of the experiment. When this was done with the non-human subjects, in the form of pigeons, they responded much like humans in that males showed much less control than females. Logue, (1995), who is discussed more below, points out that in her study done on self-control it was male children who responded with less self-control than female children. She then states, that in adulthood, for the most part, the sexes equalize on their ability to exhibit self-control. This could imply a human's ability to exert more self-control as they mature and become aware of the consequences associated with impulsivity. This suggestion is further examined below.