Systems Thinking 101
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Can you answer this?

A bathtub is draining at a steady rate. You open the tap hard, so much more water flows in than out. Then you begin closing it, slowly and steadily.

The inflow falls. For a while it is still above the outflow. At some moment the two are equal. After that, inflow is below outflow.

tap level drain

When is the water level at its highest?

The level keeps rising as long as more water comes in than goes out. That the tap is closing does not matter. The level peaks at the exact moment inflow meets outflow, and only then begins to fall.

When researchers at MIT gave versions of this problem to graduate students with strong backgrounds in mathematics, science and engineering, large numbers of them got it wrong. Not because the arithmetic is hard. Because of a shortcut so fast that the arithmetic never gets used: we assume a quantity follows the shape of whatever feeds it.

Almost everything that matters is a bathtub

  • CARBON IN THE ATMOSPHERE
    Emissions are the tap, oceans and forests are the drain. Cutting emissions is not the same as cutting the concentration.
  • CASH IN A COMPANY
    Sales are the tap. A business can grow its revenue and run out of money in the same quarter.
  • TRUST IN A TEAM
    It fills through many small deposits and drains through a few withdrawals. It cannot be injected.
  • Groundwater. Skills. Reputation. Housing. Technical debt. Nurses in a country. Each one a level that almost everybody manages by watching the tap.

Learning to look below the waterline

The four levels of the iceberg Events sit above the waterline. Patterns, structures and mental models lie below it, each level explaining more than the one above. Events What happened. You can react. Patterns What keeps happening. You can anticipate. Structures What produces the pattern. You can design. Mental models What holds it in place. You can transform.
Four levels at which any situation can be understood. Each one further down explains more of what is happening, and gives you more leverage over it.

Most of what we argue about sits above the waterline. Something happened, and we react to it. Sometimes we notice it keeps happening, and we anticipate.

Underneath is the part that produces both. The structure: the accumulations, flows and loops that generate the pattern. And below that, the beliefs that made the structure seem reasonable to build.

Systems thinking is the discipline of working below the waterline on purpose. It is a way of understanding causality, not a feeling for the whole. Structure generates behavior, so if you want different behavior, the structure is where you have to go.

It will not let you predict the future. It will let you see which futures are possible, and how your own actions make each one more or less likely.

More is coming

This is the first piece of a course on systems thinking, adapted from one taught at Pontificia Universidad Católica de Chile. Feedback loops, delays, stocks and flows, and the simulations that teach them, are being written now.

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