Did the Potato Cause the Population Boom? Sixth Graders and Causal Claims
Ms. Okafor's sixth graders are examining a table she built: rough European population estimates of about 100 million in 1650, 140 million in 1750, and 270 million in 1850; dates when potato cultivation spread in Ireland, Prussia, and France; and a note that maize yields per acre exceeded wheat. A student writes: 'The potato caused Europe's population to double because it has lots of calories.' The class splits. Ms. Okafor has studied the Columbian Exchange and wants students to learn that historical causal claims need a mechanism, correct timing, and rival explanations. She has a 45-minute period, a projector, and a textbook treatment that says the Columbian Exchange 'transformed' Europe in three sentences. Several students want to accept the claim because 'it makes sense.'
1. A student writes that the potato caused Europe's population to double. What in the claim is defensible, what is missing, and how do you respond in the moment?
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Defensible: the potato is calorie-dense per acre, grows in poor soil, is harder to loot because it stays underground, supplies vitamin C, and fits the Columbian Exchange framework - there is a real mechanism. Missing: timing, since the potato arrived in the late 1500s but spread slowly and unevenly, and the steep growth of 1750 to 1850 lags adoption by generations; rival causes, including falling mortality from smallpox inoculation and vaccination, sanitation, earlier marriage raising fertility, and other American crops such as maize; and the level of the correlation, which is continental while regions differed sharply. 'Caused' also overstates; 'contributed' is the supportable verb. In the moment, do not reject the claim - ask what we would need to see if it were true. Students should propose that potato adoption precede growth region by region, and that early-adopting regions grow sooner. Then check the table against those predictions.
2. What additional evidence would most raise or lower your confidence, and why those rather than other sources?
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Raise confidence: regional adoption or price series showing potato prices falling before population rises; comparisons of two regions that are similar except for potato timing; records showing the mechanism operating, such as earlier marriage ages or lower famine mortality in potato-growing districts. Lower confidence: evidence of growth before potato spread arrived, or early-adopting regions with no growth. These matter because they bear directly on timing and mechanism, which is exactly what a causal claim requires. They beat other options for specific reasons: the textbook sentence is a secondary summary with no evidence behind it, and a single traveler's account is an anecdote that cannot establish a trend. Also name the table's own limitation - national population figures from this period are scholarly reconstructions with real error, so 'doubled' is approximate and should be stated as such.
3. Rewrite the task so the assessment rewards causal reasoning rather than the correct conclusion.
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Present the table without a conclusion and ask students to write two competing explanations for the growth, then name one piece of evidence that would distinguish them. Or use the textbook line directly: 'The textbook says the Columbian Exchange transformed Europe - how much of this growth can we fairly credit to the potato, and what else would you need to know?' Rubric: (1) names a mechanism rather than only a correlation; (2) addresses timing; (3) names at least one rival cause; (4) states what evidence would change the student's mind. Full credit goes to a well-argued 'the potato probably contributed, but this table cannot isolate its share' and equally to a stronger claim that meets all four criteria. The point is that a task whose answer is simply 'potatoes' teaches students that history is a list of causes, which is the misconception the unit is supposed to correct.