Watching For Feathers: Threshold Effects and Imaginary Bathtubs

I’ve been thinking a lot about threshold effects, how we don’t think about them, and how much trouble that causes for humans in all sorts of ways.

Normally, we think of cause and effect as a pretty straightforward thing. I let go of a ball, it falls toward the ground. You can clearly see causality.

But cause and effect gets a little sneakier when you’re talking about threshold effects.

Imagine a bathtub. Imagine filling it halfway full with water.

Straightforward, right?

Now imagine putting giant bricks into the tub. Not so many that the tub overflows, but so that it gets really close to the top. Close enough that you can see the water tension keeping it from overflowing onto your (imaginary) floor.

Now add a single feather. That single feather is enough to finally overfill the tub, and water splashes all over your feet.

So it’s the feather’s fault, right?

Sure, it’s technically true that a feather caused the bathtub to overflow. But if you then tried to keep that tub from overflowing by paying attention to feathers and ignoring the large bricks, you’ve got a problem. Not just because you are (in this example) focusing on the smallest part of the issue, but completely ignoring other, larger contributors to the problem.

There’s a lot of these sorts of effects in our lives. Some examples:

  • Physiological responses to stress – The "life events stress scale" is an excellent example of this; once you’ve piled on a bunch of very stressful situations, even a normally minor stressor could be enough to cause a breakdown
  • Whether or not you get infected when exposed to a disease – a factor of both your own immune system, if it’s encountered the disease (or a vaccine for that disease) before, how many individual viruses or bacteria enter your body
  • Allergies – both the "achoo" kind and the epi-pen variety
  • Economic systems – think about the way government assistance frequently disappears once you hit a particular income level.
  • Other difficult to pin down multifactor issues such as Willis-Ekbom, fibromyalgia, and so on.

I admit, it’s simpler to think of things in a simple one-cause, one-effect kind of way.

It’s tempting to just look at the feather and not bother checking for the giant bricks in the bathtub.

But damn, that’s one uncomfortable bath.

Some further reading about threshold effects – including economic growth examples and advertising examples are at and respectively.

Featured Photo by Noithat rakhoi on Unsplash

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