I just got my latest Population Research Institute newsletter, which reports that "one of the nation's premier law schools" (which they do not name) is asking its students the following question:
Assume that population growth in the United States reached a point where additional growth would place a serious strain on our nation's resources, including food, water, space for housing, schools, and energy supply. Would the United States Constitution permit a law that limited people to having two biological children?
Students are then told to construct arguments for and against the constitutionality of various laws and regulations limiting childbirth, including a federal law and state laws.
A little googling hasn't turned up more information, but if any reader can identify the school, I'd be very pleased to know.
But hey, if they just put the "penalty" in the form of a "too many children tax" then I guess we're good to go, now.