Our research team has published a paper on the neural basis of the undermining effect (Kou Murayama, Madoka Matsumoto, Keise Izuma, & Kenji Matsumoto “Neural basis of the undermining effect of monetary reward on intrinsic motivation,” PNAS 107: 20911-20916, 2010) in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).
Since the undermining effect is often misunderstood, we have provided a simplified explanation here using an example of rewarding academic achievements, without losing the essence of the concept.

Motivation consists of two types: extrinsic motivation, which drives actions for rewards, and intrinsic motivation, which stems from enjoyment of the task itself. “Willingness to act” can be thought of as the sum of these two types.
If a student studies without expecting a reward and has intrinsic motivation, the equation can be represented as:

① No Reward → Extrinsic Motivation 0 + Intrinsic Motivation 1

This means that the willingness to act is 0 + 1 = 1.
Now, if we introduce a reward, such as promising ¥1,000 for scoring 100 points on a test, an extrinsic motivation to earn the reward is added. However, intrinsic motivation decreases due to the undermining effect, resulting in:

② Expecting a Reward → Extrinsic Motivation 1 + Intrinsic Motivation 0

This means that the willingness to act remains 1 + 0 = 1.
In other words, as long as rewards are given, motivation does not disappear.
However, once a reward is introduced, and studying becomes reward-driven, the original intrinsic motivation (1) is reduced to 0. As a result, in subsequent cases where no reward is promised:

③ No Reward → Extrinsic Motivation 0 + Intrinsic Motivation 0

This results in a complete loss of motivation: 0 + 0 = 0.
Thus, after going through process ②, intrinsic motivation is lost, leading to a situation where a student who once studied voluntarily will no longer do so.
On the other hand, if there was no intrinsic motivation to study in the first place:

①’ No Reward → Extrinsic Motivation 0 + Intrinsic Motivation 0

Here, willingness to act is 0 + 0 = 0, meaning the student does not study voluntarily.
If a reward for success is promised, extrinsic motivation is introduced:

②’ Expecting a Reward → Extrinsic Motivation 1 + Intrinsic Motivation 0

Now, willingness to act becomes 1 + 0 = 1, meaning the student will study as long as they expect a reward.
However, in the next instance where no reward is promised:

③’ No Reward → Extrinsic Motivation 0 + Intrinsic Motivation 0

This results in a complete loss of motivation: 0 + 0 = 0.

In summary:
If intrinsic motivation to study originally exists:

A) No Reward (Motivation 1) → Expecting a Reward (Motivation 1) → No Reward (Motivation 0)

If there is no intrinsic motivation initially:

B) No Reward (Motivation 0) → Expecting a Reward (Motivation 1) → No Reward (Motivation 0)

Previous neuroscience studies have only addressed B), where motivation is solely based on extrinsic factors.
However, in our study, we introduced an interesting task that could be intrinsically enjoyable, allowing us to examine A).
The significance of this study lies in uncovering that the cortical-basal ganglia evaluation system is involved in the process by which extrinsic motivation suppresses intrinsic motivation.

A key question for neuroscience in contributing to education is: **How can intrinsic motivation be increased, and what are the neural mechanisms that enable it?**
This remains an important research topic for the future, so stay tuned!