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Sanford and the Electoral Effect of Sex Scandals

Tuesday night’s special election in South Carolina’s First Congressional District was won by a Republican, Mark Sanford, the former South Carolina governor whose political career was imperiled in 2009 after he disclosed an extramarital affair.

As we almost always say about special elections, the race probably does not tell us very much about the national political landscape. However, it does give us another data point on how voters react to sex scandals.

It would be wrong to conclude that voters did not punish Mr. Sanford at all for his extramarital affair. In fact, a reasonable number of voters did appear to hold it against him. Last November, Mitt Romney won South Carolina’s First District by 18 percentage points. Since Mr. Romney lost the election to Barack Obama by roughly four percentage points nationwide, that means the First District is about 22 percentage points more Republican than the country as a whole.

Mr. Sanford defeated his Democratic opponent, Elizabeth Colbert Busch, by nine percentage points instead – so one quick-and-dirty estimate is that Mr. Sanford’s personal history cost him a net of 13 percentage points. It just was not enough to flip the election result in such a conservative district.

As it happens, this 13-percentage-point penalty almost exactly matches an academic analysis on how much voters hold sex scandals against candidates. A 2011 paper by Nicholas Chad Long of St. Edward’s University, which examined United States senators running for re-election from 1974 to 2008, estimated that scandals involving immoral behavior lowered the share of the vote going to the incumbent by 6.5 percentage points.

Since reducing the incumbent’s vote share necessarily increases the challenger’s vote share, that means the net effect on the margin between the candidates is twice that amount, or 13 percentage points – just as we estimated it might have been for Mr. Sanford.

This close match is a coincidence at least in part, of course. Mr. Long’s estimate reflects an average over all sex scandals in his database, but not all scandals are created equal. Mr. Sanford’s, for instance, arguably involved not only infidelity but also dereliction of duty, since he was absent from South Carolina for several days in 2009 while visiting his mistress in Argentina.

Another complication is that, other things being equal, a former governor like Mr. Sanford is overqualified to run for a House seat. Other things certainly were not equal in this case, of course – but the scandal did not rob Mr. Sanford of his political moxie.

All those qualifications aside, Mr. Long’s research would have provided a pretty good benchmark for this race – a much better one than the polls did. (I cautioned on Twitter on Tuesday afternoon that the polling is often fairly poor in House races – and suggested that Mr. Sanford should probably be considered the favorite because the First Congressional District votes so strongly Republican as a default.)

Public Policy Polling had Mr. Sanford only one percentage point ahead in a poll earlier this week – and they had Ms. Colbert Busch up by nine percentage points in a poll two weeks ago. Voters in South Carolina may not like sex scandals – but they appear to like Democrats even less.

Nate Silver is the founder and editor in chief of FiveThirtyEight.