While studying psychology, my fellow students and I replicated a famous study conducted by B.F. Skinner in the 1940s. We trained rats to press bars for rewards of condensed milk.  

We found that rats really enjoy condensed milk. Once trained, they will happily do nothing but press the bar to get the milk.  

Psychologists call this kind of training ‘operant conditioning.’ Economists have a simpler term – ‘Incentives.’ 

Rats are comparatively smart animals. But even sea slugs and fruit flies are susceptible to operant conditioning. All creatures, great and small, respond to incentives. 

Incentives are necessary, but not sufficient, for a free economy. Open markets also require understanding of free exchange and respect for property.  

Rats understand neither. They do not trade but steal with no pang of conscience. 

You might suspect that it takes a human level of intelligence to understand free exchange. But in my recent travels in India, I discovered that this is not so. 

Walking through the historic town of Vrindavan with a friend, dodging the detritus that decorates the roadside, I was surprised to feel my glasses abruptly jerked from my face. The next thing I knew, a monkey was disappearing up the side of a decrepit apartment block, my glasses in hand. 

A small mob of children appeared, excited by the drama. That attracted the attention of the adult residents, one of whom climbed onto the roof where the monkey was perched, nonchalantly chewing the silicone nose pads off my glasses.  

From there, a series of free exchanges took place, which proved the subjective nature of value. 

The monkey was persuaded to trade my glasses for a carton of orange juice. As tasty as it apparently found the silicone, it valued the juice more highly.
 
The intrepid resident returned my glasses, and I gave him 500 rupees (about $10) for his trouble. It was a small sum compared to the value of the glasses to me, but a princely one to him. We also purchased a packet of chocolates from a local vendor and distributed them to the children.  

All parties were satisfied. The monkey had orange juice, the resident had 500 rupees, the children had chocolate, and I had my glasses back, slightly worse for wear, but with a fun travel anecdote to compensate. 

Additionally, we learned that, unlike rats, monkeys understand the principle of free exchange.  

Respect for property, not so much.