The most important lesson of economics can be distilled into a single sentence:People do things because they’re worth doing.It sounds simple, but most of my columns have been about the surprising implications.When two people choose to interact, they do so because they think it’s worth it.Poor children overseas aren’t forced to work in sweatshops; they choose to because the pay is higher than their other opportunities. We might feel remorse that they have no better options, but outlawing sweatshops inhumanely takes away more options.In other past columns, I’ve defended organ auctions, free trade, Wal-Mart, and post-hurricane “price gouging” for this simple reason: People try to better themselves. Voluntary trade happens when two sides see a win-win opportunity.Increase freedom. Everyone is better off.Unfortunately, there are two exceptions to this simple formulation.Sometimes, people are wrong about what will make them happy.If people falsely believe antibacterial soap prevents disease, they’ll waste money and time on a useless habit.This is where journalism plays an important role. I hope my columns on political participation, circumcision and organic food changed preferences for the better.Unfortunately, there’s another instance where freedom doesn’t necessarily lead to the best outcome.Sometimes, the costs and benefits of our actions are borne by others. Stand up at a baseball game, and you trade a small amount of comfort for a better view. Unfortunately, you also infringe on the view of those behind you. As a result, often everyone will stand up and experience the same view they would have while sitting — except now they have to stand.If we follow our self-interests, we’re destined to perform too many of the actions that push costs on others without their permission. Pollution is the classic example.Similarly, we do too little of the actions that have spillover benefits on our neighbors. That’s why your roommates don’t wash the dishes.As I argued last year, earning a degree has career benefits for you, but it hurts everyone else by diluting the value of their diplomas. In a state of nature, too many people would go to college. In a world where the goverment uses scholarships and federal student loans to push the financial cost on others, far, far too many go to college.Economists call these nonvoluntary costs and benefits externalities, and it often leads to counter-intuitive conclusions.An STD-ridden prostitute spreads her disease to others, so the logic of negative externalities says she has too many one-night stands.As economist Steven Landsburg pointed out in “More Sex is Safer Sex,” a quiet, nervous prude who only rarely trolls for tail routinely denies others the pleasure and safety of an STD-free night. Rationally, he doesn’t consider the positive externalities, so he is far less promiscious than he optimally could be.If the shy had sex more often — but not too much — fewer people would contract STDs.I don’t expect the chaste to read this column and put themselves into the sexual line of fire. I also don’t expect companies to voluntarily cut pollution, fans to remain seated in Death Valley, or politicians to practice fiscal responsibility.Our country is in 14-figure debt because democracy gives no votes to the unborn, but democratically elected politicians can pass liabilities to them.Externalities infect our personal, business and political lives.So, there you have it. My gameplan during the past three semesters has been to encourage you to focus on win-win reactions, bring your preferences closer to what will bring you happiness and show the causes of the world’s problems.I hope it’s been worth doing.Daniel Morgan is a 22-year old economics senior from Baton Rouge. Follow him on Twitter@TDR_dmorgan.—-Contact Daniel Morgan at [email protected]