The idea that it's permissible to 'nudge' people in desirable directions has received much attention over the last few years. I've noted and commented on various incentive schemes before.
There is arguably a significant difference between offering an incentive to make one course of action more desirable and a threat or penalty to make another action less desirable. (This difference need not be one in freedom - I can agree with Steiner that threats don't reduce liberty, but they do render the agent worse off.)
It seems that some are considering a tax on junk food as a way of combating obesity. This, however, seems to me the wrong way to go about things. It ignores Mill's observation that “Every increase of cost is a prohibition, to those whose means do not come up to the augmented price; and to those who do, it is a penalty laid on them for gratifying a particular taste” (On Liberty, ch. 5: p.111 of my edition). The BBC article draws an analogy with smoking, which it suggests is taxed because it is unhealthy. There is an important difference, however, in that smoking is often harmful to others - hence the ban on smoking in public places, which is not paternalistic and can probably be justified consistently with Mill's harm principle. Arguably a tax can also be regarded as preventing (or at least reducing) harm to others, rather than to the agent him- or herself.
To ban junk food would seem to be paternalistic and thus objectionable. To tax it seems to differ only in degree.
If those in power wish to encourage healthy lifestyles, I think they should employ the carrot rather than the stick - for instance subsidizing fresh fruit and vegetables or doing more to encourage cycling and other forms of exercise.
Labels: bbc, incentives, j.s.mill, links, real life