The Punnett System

Oh Lord, I have heard the claims!

I have listened to the persuaders, the charmers, the charismatics.  Most of their ideas make sense and can follow logical patterns. (see How To Fool a Smart Person by Erik Meira)  I have a thirst for knowledge, and for universal truths… so I’m listening!

But I have one helpful tool that I use to quick-check these fantastic claims: The Punnett Square. The marriage of factors combined to make truth babies.  Hypothetical genotypes producing theoretical phenotypes. (I don’t know what all that means…)

The punnet Square is commonly used in biology (I’ll let Khan Academy teach you about that here,) but we can use it in our own evidence based practice.  Here’s how: Let’s take the claim that poor posture = pain and make a Punnet Square.

PTbraintrust Posture Pain 1

As you can see, we take the two possibilities for each variable (pain is there or not there, posture is good or bad) and mix them in.  We then look at each outcome and test it: does this outcome exist in nature? In this example, yes, all outcomes exist.

PTbraintrust Posture Pain 2

So we cannot state that poor posture equals pain, since we can easily see there are times it does not (for example the static position I am in writing this blog post…).

This does not control for Black Swan Events, but it focuses your approach to critique exciting claims.  A black swan event (often related to investment/finance) is when we assume a truth (all swans are white) because we have only seen white swans.  This is similar to a “What-you-see-is-all-there-is WYSIATI Fallacy.  Well, we found black swans in Australia… so the truth that all swans are white, was disrupted immediately.  A black swan event often comes with retrospective obviousness… for example, from what we know about different animals, don’t you think there would have been a swan that was not white somewhere?

So we in PT think: “Everyone who is coming to me with neck pain has a forward head” (confirmation bias) and we blindly follow that logic to conclude that forward head posture equals pain (confusing correlation with causation).  Until someone opened their eyes real wide and saw a person with poor neck posture outside of their clinic, asked if they were in pain and viola! a Black Swan! Now the magical claims of positioning causing pain, and society’s new-found need to see the neck-position-guru, are smashed … and the guru is quite unhappy.

I believe I digressed, but a Punnett Square may help you look at possibilities in a systematic way.  We can take Good / Bad Dead-lift Form vs Good / Bad Strength Outcomes and see what it tells us:

PTbraintrust Punnet 3

This square tells us that, again, multiple outcomes are possible. “If you want strength gains- you must have great form!” Well, maybe.  The square does not tell us that the prior statement is a lie, it is simply not the whole truth.  There must be other factors involved in strength gains. (as we know: reps, weight, nutrition, etc).

The same can be said about the Posture/Pain Punnett above: There are more factors involved… it’s not as simplistic as the charming claim makes it sound.

This method is not Level 1A research… don’t confuse its power with what we already have in place. It is just an easy, layman’s style way to check if a claim is covering all the bases. We can look at each outcome square and search for the factors contributing to each outcome.  How do these results occur?  Is there a research question here?  Has someone answered this already?

So the next time you hear a fantastical claim, or overly simplified / overly complex version of a key concept- throw it in a Punnett Square – check the outcomes for possibility.  It could be a great way to start a discussion with the claim-maker, if nothing else it is a great exercise in checking your own statements, and trying to understand the concepts around you.

“I garuantee you, ladies and gentilemen, you will like our informative posts!!!” – #PTBT

PT Braintrust Punnet 4

Matt D

4 thoughts on “The Punnett System

  1. Pingback: A Year in Review | PTbraintrust

  2. Pingback: Five Days of Fallacies: Day 1, Post Hoc | PTbraintrust

  3. Pingback: Five Days of Fallacies: Week 1, Post Hoc | Betty Roche

  4. Pingback: Five Days of Fallacies: Week 1, Post Hoc | Jerome Perkins Blog

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