and self-control? Is self-control a single faculty in charge of taming every vice, from overeating to promiscuity to procrastination to petty crime to serious aggression? If so, are there ways for individuals to boost their self-control? And could these adjustments proliferate through a society, changing its character toward greater restraint across the board?
Let’s begin by trying to make sense of the very idea of self-control and the circumstances in which it is and isn’t rational.75 First we must set aside pure selfishness—doing something that helps oneself but hurts others—and focus on self-indulgence—doing something that helps oneself in the short term but hurts oneself in the long term. Examples abound. Food today, fat tomorrow. Nicotine today, cancer tomorrow. Dance today, pay the piper tomorrow. Sex today, pregnancy, disease, or jealousy tomorrow. Lash out today, live with the damage tomorrow.
There is nothing inherently irrational about preferring pleasure now to pleasure later. After all, the You on Tuesday is no less worthy of a chocolate bar than the You on Wednesday. On the contrary, the You on Tuesday is more worthy. If the chocolate bar is big enough, it might tide you over, so eating it on Tuesday means that neither You is hungry, whereas saving it for Wednesday consigns you to hunger on Tuesday. Also, if you abstain from chocolate on Tuesday, you might die before you wake, in which case neither the Tuesday You nor the Wednesday You gets to enjoy it. Finally, if you put the chocolate away, it might spoil or be stolen, again depriving both Yous of the pleasure.
All things being equal, it pays to enjoy things now. That is why, when we lend out money, we insist on interest. A dollar tomorrow really is worth less than a dollar today (even if we assume there is no inflation), and interest is the price we put on the difference. Interest is charged at a fixed rate per unit of time, which means that it compounds, or increases exponentially. That compensates you exactly for the decreasing value of the money coming back to you as time elapses, because the decrease in value is also exponential. Why exponential? With every passing day, there is a fixed chance you will die, or that the borrower will abscond or go bankrupt and you’ll never see the money again. As the probability that this will not have happened dwindles day by day, the compensation you demand multiplies accordingly. Going back to pleasure, a rational agent, when deciding between indulging today and indulging tomorrow, should indulge tomorrow only if the pleasure would be exponentially greater. In other words, a rational agent ought to discount the future and enjoy some pleasure today at the expense of less pleasure tomorrow. It makes no sense to scrimp all your life so that you can have one hell of a ninetieth birthday bash.
Self-indulgence becomes irrational only when we discount the future too steeply—when we devalue our future selves way below what they should be worth given the chance that those selves will still be around to enjoy what we’ve saved for them. There is an optimum rate of discounting the future—mathematically, an optimum interest rate—which depends on how long you expect to live, how likely you will get back what you saved, how long you can stretch out the value of a resource, and how much you would enjoy it at different points in your life (for example, when you’re vigorous or frail). “Eat, drink, and be merry, for tomorrow we die” is a completely rational allocation if we are sure we are going to die tomorrow. What is not rational is to eat and drink as if there’s no tomorrow when there really is a tomorrow. To be overly self-indulgent, to lack self-control, is to devalue our future selves too much, or equivalently, to demand too high an interest rate before we deprive our current selves for the benefit of our future selves. No plausible interest rate would make the pleasure in smoking for a twenty-year-old self outweigh the pain of cancer for her fifty-year-old self.
Much of what looks like a lack of self-control in the modern world may consist of using a discounting rate that was wired into our nervous systems in the iffy world of our pre-state ancestors, when people died much younger and had no institutions that could parlay savings now into returns years later.76 Economists have noted that when people are left to their own devices, they save far too little for their