July 14, 2018

Rhetorical Fallacies: Cherry Picking

Writing about web page https://twitter.com/realDonaldTrump/status/946531657229701120

“Cherry picking” is when an arguer reports evidence that is favorable to their argument, but ignores valid evidence that is disfavorable. Valid argument and, more importantly, valid reasoning demands that we take all evidence into account, even when it disagrees with our beliefs and desires. Actually, it would probably be more accurate to say that valid argument and reasoning demand that take evidence that disagrees with our beliefs and desires especially into account. Finding problems and challenges to our ideas helps us make our ideas better.

President Trump’s tweet on Dec. 29, 2017 demonstrates the fallacy of cherry picking:

Strictly speaking, “East” is problematic as a location. The “East” isn’t actually a place in the United States (i.e., some cities, like Boston and New York and definitely part of the East, but cities like Atlanta or Pittsburgh are more marginal). It was also unlikely when Trump tweeted this message that every location in “the East” would have its coldest ever recorded year (and after the fact, we know this was not the case; in Boston, e.g., it got as cold as 3 degress (F) on New Year’s Eve, failing to match the record of -8 degrees of 1917). It’s also not clear what he is referring to when he says that the U.S., but not other countries, “was going to pay TRILLIONS OF DOLLARS to protect against.”

More to the point for cherry picking, though, is Trump’s implication that cold temperatures during the winter refute the reality of global warming. It was indeed very cold the day Trump tweet. According to historical data on wunderground.com, in Boston, the mean temperature on Dec. 29 was 23 degrees colder than the day’s historical average. In Washington, DC, Dec. 29, 2017 was nearly 13 degrees colder than average. But ten days earlier on Dec. 19, Boston was 9 degrees warmer than average. Boston was also 9 degrees warmer than average one month earlier on Nov. 29. Washington, DC--where Trump would be well positioned to enjoy unseasonably warm weather--was 16 degrees warmer than average on Dec. 23, 13 degrees warmer on Dec. 19, and 11 degrees warmer on both Dec. 5 and Nov. 29.

Trump can only validly claim that a single period of cold as evidence against global warming if he also admits a single period of warmth as evidence in favor of global warming. So, in Washington, DC, the week of Dec. 18-24 was unseasonably warm. If Trump were arguing fairly, he would’ve tweeted:

Global warming is making things hot for Santa. The U.S. should spend TRILLIONS OF DOLLARS to fix this! #DreamingOfaWhiteChristmas

Of course, no such tweet came from @realDonaldTrump. Trump is not actually weighing all information regarding global warming, but rather selectively tweeting cherry-picked evidence that supports conclusions he seeks. His evidence is cherry-picked, so his reasoning is invalid.

For argumentation purposes, it’s crucial to understand that dismissing Trump’s tweet as cherry picking does not inform debates about the reality of global warming. That’s a matter of climate science. But to understand and interpret the findings of climate science, we need to admit evidence in a valid and honest way. We must consider evidence objectively, and not in a manner that intentionally reaches a conclusion we’ve reached beforehand.


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