Philosophers have been debating moral values for two and a half millennia, and counting... I would argue that, at least when moral philosophy becomes political philosophy, consequences matter.
According to FBI statistics, 39% of all murders (counting only those where the murderer's age is known) are committed by young males aged 17 to 24. These murders impose great costs on the victims and, perhaps even more, on their parents, children, and other loved ones. The internment of all young males from their 17th birthday until they turn 25 would prevent these murders. Therefore, this public policy should be implemented.
Many people seem to believe that public policy should be based on this sort of reasoning, which can be expressed under the form of a syllogism:
X imposes great costs on some people;
Z would reduce the occurrence of X;
Therefore, Z must be done.
Call this the simplistic public-policy syllogism. (I was reminded of this sort of public-policy model by a Facebook comment from independent researcher Daniel Kian Mc Kiernan, which he later developed into a blog post.)
As a preliminary consideration, we want to make sure that the minor premise (the second one) of the syllogism guarantees a net reduction of X (murders committed). Z (the preventive incarceration of all young men) would not eliminate the roughly 6,000 murders committed annually by young men, for some would be committed in their prison--especially with all these testosterone-laden young guys around. But it would reduce the occurrence of murders. It would save at least some lives (if not most). As public health activists are fond of saying, "If it could save only one life..."
Even if the two premises are correct, the syllogism is logically invalid. The conclusion does not logically follow because, as philosophers know, an "ought" cannot be derived from an "is." There is a jump between the positive and the normative. A normative conclusion requires the input of moral values (often called "value judgments" by economists).
Philosophers have been debating moral values for two and a half millennia, and counting. The deontological version of ethics is usually based on some conception of natural law. I would argue that, at least when moral philosophy becomes political philosophy, consequences matter. A deontological ethics that would render everybody on earth miserable would be indefensible. Consequences, however, must still be evaluated.
We need to evaluate the consequences of a public policy compared with its absence (or with its absence in the presence of a different public policy, but I ignore this complication here). Assume that we are only interested in individuals--as opposed to, say, what Gaia thinks or what "society" feels. All the costs and benefits of Z to all individuals must be factored in. The concept of opportunity cost is useful here (as everywhere): benefits foregone are costs, and costs avoided are benefits. Thus, maximizing benefits is the same as minimizing costs over the two alternatives Z and non-Z. And, needless to say, all psychic costs and benefits must be included.
The cost of Z would include the pleasure lost by the young men during their preventive incarceration. Foregone schooling would translate into reduced future earnings and enjoyment of life. The cost of Z also includes the benefits lost by other individuals as a result of the internment policy: parents deprived of their sons during seven years, young women deprived of young men, the loss to taxpayers who have to pay for the jails and guards, and so forth.
Standard welfare economics and its practical application, cost-benefit analysis, are not capable of eliminating the need for moral criteria in evaluating the consequences of public policy. Even if the total benefits of Z are greater than its total costs, some individuals will be harmed. The idea that the individuals benefited by Z could potentially compensate those harmed, even if they did not or could not actually do it, is not sufficient, for what allows government to favor some individuals over other individuals? This is a standard and well-taken critique of the New Welfare Economics that developed in the 1930s.
Economists have demonstrated that, if everybody is to count equally, it is impossible to aggregate all individual preferences into a sort of "social welfare function" representing a moral agreement about how some can be harmed in order to favor others. I present an introduction to these proofs in my Econlib article "The Vacuity of the Political We." Somebody--individual, group, or majority-- will typically impose his value judgments on all others in society.
Add to this a crucial fact: long-term consequences that must be factored in are impossible to predict. How many young males would have become hardened criminals when they are released from their prisons, and will they commit more murders than they would otherwise have? What would be the consequences of this assault on individual liberty in general? How will all this change the nature of society and the state? The character of the jailers? The relations between boys and their families? (Would parents start aborting their male foetuses as many Chinese did to their daughters-to-be because of another sort of public policy?) Marriage and the family? Conventional morals? And so on and so forth.
To summarize, the simplistic public-policy syllogism is useless and dangerous. It is not logically valid. The fact that X imposes great costs on some people is not a sufficient reason to implement Z, because all costs must be considered. Accepting a distribution of these costs and benefits among different people requires value judgments, even if the net benefit is positive. These value judgments will have to be imposed by a majority (at best) to the rest of society. Moreover, long-run costs and benefits are impossible to forecast.