Social policy affects society most powerfully, which is inherent in the term itself, since a social policy is a policy that affects society. Let's look at a few social policies and see what a strong impact they have had.
Universal public education is a social policy that the United States has had for a very long time, almost since the beginning of the nation. The citizens of each state agree to pay for every resident of the state to receive a public education, usually from kindergarten all the way through grade 12. The impact of this is literary and numeracy rates higher than most nations, although not as high as either could or should be, a citizenry that is at least theoretically prepared to enter the work force or college, better prepared to vote intelligently, and productivity rates that are among the highest in the world. Public education provides fuller employment. It provides a stronger middle class, which is the glue that holds a democracy together. This has been, for the most part, one of the most successful social policies this country has.
Racial integration is another social policy, one that has had mixed results. Its beginning, I would say, was when the Supreme Court decided Brown v. Board of Education (1954) and when Congress finally passed the Civil Rights Act of 1964. Both were legal manifestations of a social policy meant to integrate our very segregated society. The implementation of this policy was by no means smooth, with federal marshals on hand to protect African-American children attending white schools for the first time, for example. Federal regulation did not always reinforce the policy, with housing, for example, remaining segregated for a very long time before anyone chose to do anything about it. In employment, integration has been a more successful endeavor, since the Equal Employment Opportunity Commission has acted to enforce statute and regulations more reliably. Sadly, the Supreme Court has relinquished much of its jurisdiction over school segregation, and in many school districts, segregation is now the rule again, rather than the exception. I would have to say that racial integration as a social policy has a long way to go to have the impact upon society that it should be having, which is that we judge one another "by the content of our character" (King).
Home ownership is a social policy, too, promoted in most presidential administrations, carried out through fiscal policy, such as the tax code, and various governmental loan programs that have made it easier for people to buy homes. With the exception of the housing bubble that led to the recession of 2008, this has been a fairly successful social policy. Its impact has led to greater home ownership, which leads to more stable neighborhoods, better maintenance of property, equity that can be drawn upon, and assets that can be passed on. The housing bubble was the result of too much promotion of the social policy, leading lenders to approve mortgages for people who could not afford them, creating millions of homeowners who could not manage to hang onto their homes. Greed was a factor, too, since lenders were more concerned about making money on the mortgages than they were with whether or not people could afford to buy a home. The government's lack of regulation of this sector of the economy nearly decimated the impact of what is actually a good social policy, rendering large parts of society homeless and in debt.
I think the moral of the story about social policy is that it is not enough to have a court decision, statutes, or government agencies. If a social policy is to be effective, it must have the full force of government, from top down, promoting it. If one part of government is promoting it, while another part of government is shooting it in the foot, no social policy, no matter how good it is, is going to work.