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Unintended consequences
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Unintended consequences

In the 1950s the WHO sprayed part of Borneo with DDT to combat malaria. This response set off a chain of unintended consequences.

In the 1950s the WHO sprayed part of Borneo with DDT to combat an outbreak of malaria that was affecting the Dayak people. According to this video, a chain of unintended consequences followed:

“In the early 1950s, there was an outbreak of a serious disease called malaria amongst the Dayak people in Borneo. The World Health Organization tried to solve the problem. They sprayed large amounts of a chemical called DDT to kill the mosquitoes that carried the malaria. The mosquitoes died and there was less malaria. That was good. However, there were side effects. One of the first effects was that the roofs of people’s houses began to fall down on their heads. It turned out that the DDT was also killing a parasitic wasp that ate thatch-eating caterpillars. Without the wasps to eat them, there were more and more thatch-eating caterpillars. Worse than that, the insects that died from being poisoned by DDT were eaten by gecko lizards, which were then eaten by cats. The cats started to die, the rats flourished, and the people were threatened by outbreaks of two new serious diseases carried by the rats, sylvatic plague and typhus. To cope with these problems, which it had itself created, the World Health Organization had to parachute live cats into Borneo.” – http://catdrop.com.

The sign graph below summarises the World Health Authority’s view in terms of the relationships between the Dayak malaria problem, number of mosquitos, WHO sprays DDT, and the World Health Organisation.

sign diagram showing DDT killing mosquitos and relieving the malaria


The following phrases describe parts of this system: number of wasps; number of caterpillars; destructions of thatched roofs; number of DDT contaminated insects; number of cats; number of rats; and the number of unhappy people.

Use these phases to extend the sign graph above in the space on its right.

When you have finished, compare your sign graph with ours in the next step.

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Systems Thinking and Complexity

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