Explain how an understanding of the concept of unintended consequences affects policy making

When government enacts a new law or regulation, it often has a different effect from what was initially intended. Some of these outcomes can be positive. For example, there has been a great deal of research into the unintended consequences of policies designed to improve social or economic outcomes for disadvantaged segments of the population. Policies that are intended to reduce poverty often have what Berkman (2011; see the source below) calls "spillover effects." Research shows that the Earned Income Tax Credit (EITC) in the United States improves the probability that a single mother is working, reduces the probability that a mother is smoking, and increases birth weights of infants. In South Africa, if a woman receives a pension, it improves the health of her granddaughters living with her. In other words, policies designed to improve the economic well-being of one segment of the population can have unintended consequences and improve the well being of their children or other members of their households in different ways. 
However, sometimes policies have negative unintended consequences. For example, research conducted by Peltzman (1975) showed that seat belt regulations in states resulted in people wearing their belts, as the law intended. However, traffic fatalities did not go down, as people who wore seat belts drove less safely, and while they were protected by their seat belts, pedestrians and cyclists were not.
Therefore, when policy makers are considering laws and regulations, they must do more research into the unintended consequences that might result from that legislation. This type of review should be part of the research conducted before the law is passed. 
Sources:
Berkman, Lisa F. "Unintended consequences of social and economic policies for population health: towards a more intentional approach." Eur J Public Health (2011) 21 (5): 547-548. DOI: https://doi.org/10.1093/eurpub/ckr124 Published: 24 September 2011.
Peltzman, Sam (1975). "The Effects of Automobile Safety Regulation." Journal of Political Economy Vol. 83, No. 4 (Aug., 1975), pp. 677-726. Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/1830396.