I had just eaten a little plastic carton of yogurt and I tossed it into the recycling bin. She said “That yogurt carton needs to be rinsed before you can recycle it.” And I thought to myself “That can’t be true. First of all, the recyclers are going to clean whatever they get before they start processing it so it would be a waste for me to do it here. Plus, the minuscule welfare gains from recycling this small piece of plastic would be swamped by water, labor, and time costs of rinsing it.” I concluded that, as a matter of policy, I will not rinse my recyclable yogurt containers.
So I replied “Oh yeah you’re right.”
You see, I didn’t want to dig through the recycling bin and rinse that yogurt cup. By telling her that I agree with her general policy, I stood a chance of escaping its mandate in this particular instance. Because knowing that I share her overall objective, she would infer that was that my high private costs of digging through the recycling that dictated against it under these special circumstances. And she would agree with me that letting this exceptional case go was the right decision.
If instead I told her I disagreed with her policy, then she would know that my unwillingness was some mix of private costs and too little weight on the social costs. Even if she internalizes my private costs she would have reason to doubt they were large enough to justify a pass on the digging and rinsing and she might just insist on it.