The Mousetrap Analogy
Trapped by Design

In their continuing efforts to make a case for the "intelligent design" of biochemical systems, Michael Behe and others have compared such systems to mousetraps. By invoking an ordinary object like the mousetrap, they hope to explain their concept of "irreducible complexity" in terms that people can easily grasp. Here's an example of how this comparison is made:

"A good example of such a system is a mechanical mousetrap. ... The mousetrap depends critically on the presence of all five it its components; if there were no spring, the mouse would not be pinned to the base; if there were no platform, the other pieces would fall apart; and so on. The function of the mousetrap requires all the pieces: you cannot catch a few mice with just a platform, add a spring and catch a few more mice, add a holding bar and catch a few more, All of the components have to be in place before any mice are caught. Thus the mousetrap is irreducibly complex." [MJ Behe, 1998, "Intelligent Design Theory as a Tool for Analyzing Biochemical Systems," in Mere Creation, p. 178]

No Simpler Mousetrap?

(Below) John MacDonald's drawings of a standard 5-part mousetrap, following by a series of simpler mousetraps with 4, 3, 2, and finally, just 1 part.

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As is true with many of the arguments marshalled against evolution, it's attractive, compelling, and easily made. And it's completely wrong.

 The Mousetrap Argument has a Fatal Flaw
 If parts of a mechanical machine like a mousetrap can be used for different purposes, then portions of any "irreducibly complex" biochemical machine can be fully-functional in other biochemical contexts as well. This means that Behe's argument against evolution fails, because the entire machine need not be assembled for natural selection to produce its individual parts.

A series of drawings made by John MacDonald at the University of Delaware (left) show just how quickly the assertions behind Behe's mousetrap analogy collapse upon inspection. It it remarkably easy to construct a mousetrap with just 4 parts, 3 parts, or even a mousetrap with just one part. As MacDonald himself was careful to note, none of these contraptions are nearly as good as the standard 5-part mousetrap, but that's exactly the point. Working mousetraps don't have to have each of the 5 standard parts to be functional. If they have fewer parts, they can still be made to work.

Michael Behe was so concerned about MacDonald's drawings (and my use of them in a talk in June of 2000) that he wrote a specific rebuttal called "A Mousetrap Defended."

As is usual with Behe's rebuttals, it completely misses the point of the argument. Behe argues that MacDonald's four simpler mousetraps do not present a good model of a "Darwinian process." Even the simplest mousetrap, Behe argues, requires "the involvement of intelligence," and the "involvement of intelligence at any point in a scenario is fatal."

I agree. And if I or MacDonald or any one else had ever presented the simpler mousetraps as examples of an evolutionary transition, Behe would be right.

However, that's not the point of the argument.

Rather, MacDonald's drawings address Behe's contention that "all components have to be in place before any mice are caught." They don't, of course, because there is more than one way to construct a mousetrap from mechanical parts. Why is this worth pointing out? Because Behe uses the mousetrap analogy to "prove" that biochemical machines, also composed of multiple parts, could not possibly have originated from simpler assemblies. The simpler assemblies don't work, according to Behe. But MacDonald has shown that simpler mousetraps do work, and therein lies the danger to Behe's ideas.

If simpler versions of this mechanical device can be shown to work, then simpler versions of biochemical machines could work as well ... and this means that complex biochemical machines could indeed have had functional precursors... something that Behe asserts could not have happened.

In spending so much energy showing that this sequence of mousetraps could not have "evolved," Behe misses the whole point of MacDonald's demonstration. In fact, he also overlooks the most important error in his mousetrap argument, which is his contention that function is abolished by removing any part of an "irreducibly complex" system. At the very same conference, I removed two parts from a mousetrap (leaving just the base, spring, and hammer), and used that 3-part device as a functional tie-clip. I then detached the spring from the hammer, and used the device as a keychain. If I had cared to, I might have used the base and spring (2 parts) as a paper clip, my tie clip (glued to a door) as a door knocker, the catch as a toothpick, or the base as a paperweight.

As these examples show, portions of a supposedly irreducibly-complex system may be fully-functional in other contexts, and this is the biologically relevant part of the argument. Behe argues that natural selection cannot favor the evolution of a non-functional system (which is true), and then argues that no portion of an "irreducibly complex" system (such as a mousetrap) could have any function. As my 3-part tie clip shows, that's false, and it's false in a biologically-relevant way. If portions of a multipart biochemical are useful within the cell in performing other useful functions, then evolution has a perfectly reasonable way to put the parts of such machines together. This is, incidentally, exactly the case for the very systems that Behe cites. The microtubules, cross-bridges, and linking proteins of the eukaryotic cilium (to use one of his favorite examples) each have other functions within the cell that would favor their production by natural selection.

In showing that it is possible to use part of a mousetrap for a different purpose, one shows by analogy that it is also possible to use part of a biochemical system for a different purpose. That's the fatal danger of the mousetrap analogy for Behe's argument, and it has become a trap from which he cannot escape.

Kenneth R. Miller
Professor of Biology
Brown University
Providence, RI 02912