Argument from adverse consequences

From RationalWiki

An argument from adverse consequences uses a possible negative outcome (the premise) as a reason for the conclusion that discounting a particular behavior is desirable. An example may go like this: teaching evolution may lead to some people theorizing eugenics, and eugenics is bad, therefore teaching evolution is bad.

In general, the power of this argument is directly proportional to the strength of the premise: for example, a proposal to discontinue a particular medicine because there are mild adverse effects (e.g. hair loss) in a small proportion of the cases would generally be considered dubious, while discontinuing the medicine because serious adverse effects (e.g. early death or deformed babies) would generally be considered reasonable.

Adverse consequences can be found in nearly all matters of life -- for example, Christianity, which has has passages extolling the virtues of peace, patience, meekness and 'turning the other cheek' in its holy texts, also has encouraged the Crusades and Inquisition, in the past, and can thus be seen as bad. One can fall back on the No True Scotsman argument and suggest the Crusades were not enacted by true Christians, and therefore do not reflect badly on Christianity.

Since most acts will have both good and bad consequences, many of them unforeseen and unintended, it requires great care to deploy the argument from adverse consequences properly. The most frequent logical fallacy in this regard is when relatively small adverse effects are used to argue against practices with highly significant positive outcomes, e.g. that racial integration is bad because we no longer maintain separate bathrooms and water fountains which results in job losses for plumbers. Another way this is applied fallaciously is in arguing whether something is true or not. Just because something is perceived as having adverse consequences if it is true, does not make it suddenly become untrue. This is a form of wishful thinking. Just as when something is perceived as having good consequences if it is true, this perception does not actually make it true.