<p>Not only can it take many years to complete a doctoral degree, but at least in the sciences additional postdoc research and training is very common. That is a lot of years with little or no income. Of course, later on salaries are likely to be substantially bigger than without the advanced degree. Then there are progressive taxes. With high tax rates on that extra income, it makes it very difficult to recover from years of little or no income. Our society does not make it easy to go on to an advanced degree. You need to do it for love, not money.</p>