Preferences switch as the distant becomes instant. 5  Weber (1958). 6  Franke et al (2007). 7  Levine (1997). 8  Thaler and Shefrin (1981). 9  McClure, Laibson, Loewenstein and Cohen (2004). 10  For example, Pigou (1920), Ainslie and Haslam (1992) and Laibson (1997). BIS Review 114/2010  3  In these situations, the double-self becomes a practical reality for both animals and humans. Experimental evidence lends strong support. Hyperbolic discounting has been found in such diverse case studies as pigeons pecking corn, students choosing video games, women choosing anaesthesia in childbirth and drug addicts assessing their next fix. 11 Psychiatrically, it may emerge as multiple personality disorders. Addiction is, in some respects, hyperbolic discounting writ large. 12 It is Pigou’s defective telescope being held the wrong way round. Immediate gratification (drugs, alcohol, sex, food) is sought irrespective of the long-term consequences. There is a neurological explanation for such behaviour. Drugs and some foodstuffs flood the brain with dopamine, which stimulates the midbrain associated with instant reward. This might explain why addictive behaviour appears to be more prevalent in humans than in animals. While few squirrels are dieters, even fewer are glue-sniffers. Impatience has implications for economic and financial systems every bit as profound as its well-behaved twin. For economies, hyperbolic discounting would tend to generate undersaving, as people seek instant gratification. And neo-classical growth theory says that undersaving has adverse implications for long-run investment and growth. Some jam today would come at the expense of a whole jam-jar tomorrow.

Self-destructive cycles Just as patience can self-generate, so too can impatience. And while patience generates self-improving cycles, its alter ego can create self-destructive cycles. Addiction is the classic self-destructive cycle. Drugs and alcohol chemically alter the balance of the double-self, increasing the value of instant gratification. This shortens time horizons, increasing further the value of instant gratification in a downward spiral. Unless arrested, this unfulfilling equilibrium becomes self-fulfilling. 15  For example, Guven (2008). 16  Charness and Gneezy (2009). 17  Christakis and Fowler (2009). 18  For example, Tuckett (2009). 19  For example, National Institute on Drug Abuse (2007). BIS Review 114/2010  5  Similar cycles have been found with a more commonly-prescribed drug – work. Workaholism arises from a spiralling imbalance between work and life. 20 Too much work lowers the value of life away from work. This encourages even greater work and an even greater work/life imbalance in a downward spiral. Obesity can likewise generate self-affirming negative behaviours, often operating through low self-esteem. As with self-improving cycles, these self-destructive spirals have a social dimension. Bad behaviours rub off just as much as good, with three degrees of influence. It is not just happiness, diet tips and gym membership cards which pass between friends’ (friends’ friends). So too do depression, chocolate cake recipes and membership of addiction clinics. 21 As patience is contagious, impatience is infectious. So self-destructive cycles in social systems are the result of a neurological seed nurtured by sociological forces. Self-destruction, like self-improvement, is part nature, part nurture.