Weber

Like many doctors, I dread the moment I give my own child the vaccine jab. It's not hard to see why: we are supposed to safeguard our kids but there won't be pain-free needles.

The task of giving Jasmine a quick stab wasn't that difficult. When my daughter learned about the seasonal flu jab, she didn't run away. My wife held Jasmine and I picked the right spot - and voilà - a peaceful indentation of her thigh. After that my wife asked me if I wished to give her the hepatitis A vaccine as well. We purchased the vaccine few months ago and waited. And waited. And waited. And it has remained in our fridge. I hemmed and hawed for a while, before I decided to postpone the second injection. Of course I didn't have a whiff of evidence to support my claim to separate the two injections.

If the idea of separating two injections seems humane at first, it doesn't once you know the psychological principle known as "Weber's law," named after the nineteenth-century German physiologist Ernst Heinrich Weber. Although most medical students should be familiar with his Weber test (a screening test for hearing), we might have overlooked the Weber's law. Weber's law states that the impact of a change in the intensity of a stimulus is proportional to the absolute level of the original stimulus. Loosely speaking, you must shout to be heard in a noisy environment whereas a whisper works in a courtroom.

It also means that the change in any stimulus matters less and less with every increase in the absolute level of that stimulus. The idea behind Weber's law is that if you have a number of dental cavities to be filled, get them all taken care of in one trip to the dentist. Unsettling as it is, the pain of two moderately bad experiences will typically exceed the pain of experiencing both at one time.

Which means, come to think of it, that I should have given my daughter two shots at one go.