Expecting a longer life, happy people defer immediate gratification and save. In consequence, they enjoy a more prosperous tomorrow as they harvest the fruits of their investment. In these models, happiness is not just fulfilling; it is self-fulfilling. Exercise and dieting generate similar self-improving behavioural cycles. Take gym attendance. A recent study found that providing a small financial inducement to attend a gym generated radically different fitness behaviour. 16 It served as a catalyst for an exercise regime which, once begun, was much more likely to be completed. Improving health improved life chances, inducing long-sighted behaviour which was then both fulfilling and self-fulfilling. While self-improving cycles shape individual behaviour, they can also have broader social consequences. Studies have found strong behavioural contagion not just between friends, but between friends’ friends and indeed friends’ friends’ friends. In other words, people’s behaviours have “three degrees of influence”. 17 Sociological contagion has been found in studies of happiness, exercise and dieting. It may have a neurological explanation, as imaging suggests we often incorporate friends’ views and attitudes into our brains as if they were our own. 18 Latterly, models of economic systems have got in on the act. Growth, too, can be selfimproving or endogenous – for example, because investment in human capital is selfreinforcing. 19 Working, like dieting, is good for self-esteem and long-term productivity. Idling, like over-eating, is bad. History suggests a strongly self-reinforcing relationship between saving and growth. That is probably why cross-country studies point to such strong growth enhancing effects of financial deepening.

The squirrel does not store food as a dietary device. The patience “gene” The source of human self-command, or patience, appears to be neurological. Recently, this has become detectable using brain imaging technology. That suggests the pre-frontal cortex of the brain is key to self-control and patience in humans. 2 The pre-frontal cortex develops with age, being least developed in small children. In other words, neurology has confirmed the suspicions, and frustrations, of generations of parents with impatient toddlers. The pre-frontal cortex is also the part of the brain which has developed most recently. Human skulls developed from prominent brow ridges of Neanderthal man to the high straight foreheads of today’s homo sapiens to accommodate the growth of the pre-frontal cortex. That tells us the patient part of the brain was close to being fully developed perhaps around 30–50,000 years ago. Today, although the pre-frontal cortex in humans is no larger than in a gorilla, its evolution has resulted in patience having become a distinctive, perhaps unique, human attribute. As such, it is not too big a leap to imagine patience having had important implications for the evolution of human systems, economic and social. Take economic growth. The NeoClassical growth model in economics provides a simple link between patience and development. Patience generates savings by households which in turn finances investment by companies. And capital accumulation by firms, growth theory suggests, is then the prime driver of future output. Countless empirical studies, across time and countries, have confirmed that theory.