By Julia Holmes for Men's Journal When men hit their 40s, their happiness hits the skids. That's just one of the insights that Paul Dolan, a professor of behavioral science at the London School of Economics, has found in his 10 years of studying what makes us happy. In his new book, Happiness by Design: Change What You Do, Not How You Think, Dolan lays out simple solutions for increasing life satisfaction: Structure your days around the things you enjoy, stop toiling away toward goals you may not even want to meet and balance your life with purpose and pleasure. You slam positive thinking.