Are Temperature and Incentives Compliments or Substitutes? Evidence from 1947

Here's another gem from the lab of Norman Mackworth (which in my head is looking increasingly like the Dharma Initiative): "High Incentives Versus Hot and Humid Atmospheres in a Physical Effort Task" N. H. Mackworth (British Journal of Psychology, 1947).  The setup is very similar to my last post, except this time the workers are doing arm curls until complete exhaustion:

But Mackworth varied whether the worker was encouraged to push themselves to do more (verbally and visually).  In all cases, the workers with encouragement did more arm curls.  But at high temperatures, this margin dropped substantially (see fig). In this case, it seems like moderate temperatures and incentives are compliments.  This sounds like unfortunate news for hot developing countries...

[For a review of what's happened since 1947 in this field of ergonomics, see the book chapter here.]