The grapes and cucumber experiment is a rare example of behavioural research that has such obvious and wide-ranging implications that it's crossed over into popular culture.
In this experiment, two monkeys in cages next to each other perform a simple task, like handing a pebble to a researcher, and are rewarded with pieces of cucumber. They're both happy to do the task, no bother.
Then the researcher starts giving grapes to one of them. The monkey who keeps on getting cucumber as the reward has a tizz, throws the cucumber, and stops doing the task.
The popular lesson of this experiment, which has been replicated numerous times, is that fairness is part of our biological heritage; it's not something 'made up' by dissatisfied socialists.
Another way to look at it is that relativity matters. It's not the absolute value of the reward that's going to determine whether a task (or job) is seen as worth it, but how it compares with what others are getting too.
What's this got to do with teachers?
Over the past 10 years teachers have kept on getting cucumber while other workers are getting grapes. Relativity between what teachers earn and other workers has seriously declined.
A simple measure of this is the comparison between secondary teacher salaries and the median earnings of all employees. Since the early 2000s we've dropped significantly, from 1.8 times the median (for experienced teachers on the top of the pay scale) to just over 1.5. In effect, this means jobs that used to earn less than teachers now earn more, and jobs that require less than our four years' study are catching up.
The previous Government was very keen to say teachers' earnings on its watch had kept up with inflation as measured by the consumer price index (CPI). There are problems with this.
One problem is that the CPI is based on a basket of goods that most people, low to middle income earners, simply don't buy very often. Yep, flash TVs and new cars have got relatively cheaper over recent years, keeping CPI low, but basic costs like housing and food haven't. It's accommodation costs in particular that take up the bulk of regular workers' incomes, and teachers', and we all know what's happened to them in the past decade.
Another problem with inflation is that it doesn't measure the size of the pebbles the monkeys hand over for their cucumber. The work that secondary teachers do has become far more complex. Just in the past few years paper-work demands have exploded. There are now forms to fill out when you confiscate a student's phone, or go on a class trip, not to mention the planning, marking and moderation requirements of NCEA.
With the social trend away from automatic respect for authority figures to a more liberal and sceptical approach (which is largely for the better), teachers have come up with new ways to make their classrooms productive environments. Those pebbles have just kept on getting bigger and bigger.
The effects are stark. At current loss rates, and without factoring in the growth in student numbers over the next 10 years, we need about 1400 new teachers in secondary schools each year.
We've gone from averages of more than 1100 secondary teachers graduating each year in the mid to late 2000s to just over 700 in 2016, with serious declines year on year from 2011 onwards. That means we're training enough teachers to replace half the number who are leaving.
The biggest declines have been in the number of career changers coming into teaching. Not many people come straight out of university and into secondary teacher training, and that has probably been good for students over this time, being taught by a wider range of people with different experiences. Career changer numbers have halved in a decade.
Despite all this, the emotional rewards of secondary teaching are high. Bringing out the best in students, seeing their eyes light up with new understanding, is a bonus few careers can offer. This is where the analogy to the monkey in a cage handing over a pebble definitely stops.
But there are some realities that the new Government will have to face if it wants to live up to its promise of delivering great educational opportunity to all Kiwi students. The relative attractiveness of the career matters, and at the moment it's clearly not providing the grapes.
• Jack Boyle is president of the Post-Primary Teachers' Association.