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Red Grapefruit and the Half-Life of Learning


Eating red grapefruit for breakfast this morning made me think about the half-life of learning.  The grapefruit reminded me of a science project that I did for my high school chemistry class 50 years ago.  I set about to determine the pH values of red vs. yellow grapefruit.  It was a trivial project.  Nonetheless, I still remember it a half-century later, which is far from trivial.

I don’t remember much else from Mr. Gruhn’s chemistry class, apart from his passion for science and pride in placing his students at Caltech and Harvey Mudd.  I no longer remember Mr. Gruhn’s lectures on how to balance chemical equations or his instruction on how to perform titrations. But I still remember, after all these years, the little experiment I designed and carried out to test the acidity of grapefruit. 

I suspect this is partly because the project connected to something I cared about: fruit!  And partly because I had to explain what I learned to others during a science fair.  We learn by teaching— lernen durch lehren, as they say in German.  But mostly because the project engaged me in active learning: it required me to ask questions, formulate a hypothesis, conduct experiments, analyze the results, and write up my findings.  

All these strategies are known to lengthen the half-life of what is learned. All are congruent with the BYU–Hawaii learning model: prepare, engage, improve.

The half-life for most of what we learn in school is depressingly short—certainly nowhere near 50 years.  Like a radioactive isotope, what we learn is constantly decaying.  We go about forgetting.  And the rate of decay (or forgetting) is initially very rapid, even exponential. It can be graphed like this:

graphThis famous “forgetting curve” was first formulated by a German psychologist named Hermann Ebbinghaus in the late nineteenth century. It expresses the rate he forgot nonsensical combinations of letters.  Ebbinghaus discovered that he forgot most of what he learned and forgot it very rapidly—the curve is very steep within the first few hours. His forgetting curve has been verified in many subsequent experiments.

Because students forget so much so quickly, I am always curious about the half-life of my own classes.  When I meet former students, I often ask them what they remember from class. Most often my students mention coming to my home (I always had classes to my home at least once a semester), or memorizing a poem (sometimes they can still recite it), or performing a scene from Shakespeare, or writing a research paper. 

In other words, they remember what they did, especially if it was a little out of the ordinary classroom routine. As Susan often reminds me, people are more likely to remember what they say than what you say. You can check this observation against your own experience.

Occasionally, however, my students comment on something I said in class.  Often it is something that had special spiritual significance for their lives. For example, a couple of students from different classes mentioned the impact of class discussions about Abdiel in Milton’s Paradise Lost, the faithful angel who alone chose to follow God when all his fellows chose to follow Satan.  One told me that Abdiel’s example of moral courage impressed him so much that he named his son Abdiel.  Another told me that it helped her have the courage to choose to have children when others criticized her for making this choice.  The Spirit can greatly increase the half-life of learning.  We remember when our hearts and minds burn so intensely with the Spirit that the classroom becomes holy ground.

Most students also remember my enthusiasm for the subject, as I do Mr. Gruhn’s for chemistry, even if they as I have forgotten the particulars of the class content. We teach what we are and what we love.  As Wordsworth said, “what we have loved, / Others will love, and we will teach them how” (The Prelude). We long remember our teachers’ love for the subject and, perhaps even longer, their love for us.  The love of a teacher for subject and student can have a very long, potentially eternal, half-life.

And sometimes something as simple as red grapefruit can bring it all back.

John Tanner

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