Why does a delay between a rat pressing a lever and receiving food make learning harder?

In instrumental conditioning, learning is based on the temporal relationship between a specific behavior—such as a rat pressing a lever—and the subsequent delivery of a reinforcer, such as food.1

A delay between the rat's action and the arrival of the food makes learning significantly harder because it interferes with the animal's ability to establish a clear association between the two events. Conditioning is most effective when the response and the reinforcer are closely linked in time, as this allows the animal to identify the behavior as the cause of the reward.12

When a gap is introduced, the animal may struggle to attribute the food to its own action, especially if other environmental stimuli occur during the interval that might be perceived as better predictors of the reward. While animals can sometimes learn despite delays, the effectiveness of the conditioning typically declines as the interval increases, as the temporal contiguity is a fundamental factor in how organisms discover the causes of significant consequences.21