Which reinforcement schedule involves reinforcement for the first response after a fixed period?

The correct answer is the fixed-interval schedule. This reinforcement schedule delivers reinforcement for the first response that occurs after a predetermined, consistent amount of time has passed. For example, if a reinforcement occurs every 5 minutes after a behavior is performed, a subject will receive reinforcement (such as food, praise, or a reward) as soon as they engage in the desired behavior after that 5-minute period.

This schedule is particularly interesting because it leads to a "scalloped" response pattern; subjects may show a slight pause after receiving reinforcement and then gradually increase their response rate as the time for the next reinforcement approaches.

In contrast to the other options, a variable-ratio schedule provides reinforcement after an unpredictable number of responses, making it very effective for maintaining high response rates. A variable-interval schedule reinforces the first response after varying amounts of time, which results in steadier rates of response over time compared to fixed schedules. Continuous reinforcement, on the other hand, provides reinforcement every time the desired behavior occurs, which is very effective for behavior acquisition but does not involve the time constraint that characterizes the fixed-interval schedule.