  These concerns about differential access to the benefits of genetic technologies and the likely social consequences of such differential access have been addressed by bioethicists and moral philosophers from perspectives informed by competing theories of distributive justice. Libertarians support the “genetic supermarket” famously envisioned by Nozick, where prospective parents and resulting market forces determine which genetic modifications are offered—not centralized decisions made by biologists or government. It has been noted, however, that what is harmless or beneficial in individual cases (e.g. sex selection, increased longevity, competitive goods like beauty or intelligence) may be neutral or even harmful for society taken collectively (Kavka 1994). And yet, given that the state does not restrict parental autonomy when it comes to environmental boosts to their children's abilities (e.g. private schools, music lessons), it is argued that there is no principled distinction between genes and environments which would override a similar right to genetic enhancements (Buchanan et al. 2000). Liberals have used Rawlsian principles to urge a variety of state-imposed limits on a free market in genetic technologies (Brown 2001; Buchanan et al. 2000; Daniels 1994; Farrelly 2002; Resnik 1997b). In their 2000 book From Chance to Choice: Genetics and Justice, in order to protect equal opportunity, Allen Buchanan, Dan Brock, Norman Daniels, and Daniel Wikler argue that the state should ensure access to a “genetic decent minimum” which includes interventions directed to natural primary goods—whether treatments or enhancements—insofar as these are consistent with the neutrality of liberal democracies concerning conceptions of the  good.[62] Parental choices would be limited only where these are inconsistent with a child's right to an open future, carry a risk to public good (e.g. sex selection resulting in societal imbalance), diminish equal opportunity if available only to the comparatively well off, or involve enhancements with “positional advantages” (e.g. height) that would be self-defeating if widely available and unfair—and contrary to Rawls’ difference principle which requires permissible inequalities to work to the advantage of all, especially the worst off—if not, and where enhancements would lead to the disabling of others by raising the bar for what is required to participate in society's “dominant cooperative  scheme.”[63]