In Knox v. SEIU, Breyer dissents with Kagan, and challenges the Court’s decision:

The debate about public unions’ collective-bargaining rights is currently intense. The question of how a nonmember indicates a desire not to pay constitutes an important part of this debate. Must the union assume that the nonmember does not intend to pay unless he affirmatively indicates his desire to pay, by “opting in”? Or, may the union assume that the nonmember is willing to pay unless the nonmember indicates a desire not to pay, by “opting out”? Where, as here, nonchargeable political expenses are at issue, there may be a significant number of represented nonmembers who do not feel strongly enough about the union’s politics to indicate a choice either way. That being so, an “opt-in” requirement can reduce union revenues significantly, a matter of considerable importance to the union, while the additional protection it provides primarily helps only those who are politically near neutral. See generally Sunstein & Thaler, Libertarian Paternalism is not an Oxymoron, 70 U. Chi. L. Rev. 1159, 1161 (2003) (explaining that default rules play an important role when individuals do not have “welldefined preferences”). Consequently, the Court, which held recently that the Constitution permits a State to impose an opt-in requirement, see Davenport, 551 U. S., at 185, has never said that it mandates such a requirement. There is no good reason for the Court suddenly to enter the debate, much less now to decide that the Constitution resolves it.

The Constitution does not enact Mr. Sunstein’s social statics.

And in Southern Union v. US, Breyer also talks about nudging:

Today’s holding, by unnecessarily complicating the trial process, may prove workable only because it nudges our system slightly fur­ ther in this direction. I see no virtue in doing so.