First, to be entirely honest, there's no real way to tell. The government really doesn't track that kind of thing, and even if they did, their data would probably be inaccurate, as this particular problem is widely under-reported, according to most experts (and we won't even touch on the problem of false memory syndrome and how it might skew the results).
The argument can still be made, of course. There are a number of fascinating medical anomalies in South Carolina, any or all of which could be genetic, and thus reinforced by inbreeding: South Carolina leads the country in stroke deaths, and has the third highest rate of infant mortality in the United States; epilepsy, which affects between 0.4% and 1% of the rest of the country (lifetime prevalence rate), affects 2.2% of South Carolinians.
Interestingly, they also have one of the highest rates of HIV infection in the country, but that probably isn't related. (Not directly related, anyway...)
Having said that, though, we are still left with one question: what the hell is wrong with South Carolina?
South Carolina has a long history of racism - they were so in love with their plantations and ability to chain up fellow human beings that they were the first state to secede from the Union; and Edmund Ruffin was the South Carolinian (technically, a transplated Virginian) who claimed to have fired the first shot at Fort Sumter, kicking that whole mess off. (Also, upon hearing that General Lee had surrendered at Appomattox, Ruffin penned a letter stating his hatred for "the perfidious, malignant and vile Yankee race," and shot himself in the head, which many say gave him the dubious honor of firing both the first shot and the last shot of the Civil War.)
South Carolinian John C. Calhoun (7th Vice President of the United States) even gave a speech on the Senate floor where he explained that slavery was a "positive good" - he also spearheaded a gag rule that automatically tabled any discussion of slavery in the Senate.
"But, oh, Nameless One," you're probably saying, "that's the past. What about the present?" But is it truly the past? I'm not so sure, because when Mike Huckabee, while talking about the Confederate "stars and bars", can say to a group of South Carolinians, "You don't like people from outside the state coming in and telling you what to do with your flag," the audience just cheers, and nobody feels constrained to say, "Uh, Mike, that isn't our flag any more."
Regardless of that, though, South Carolina is well known for having more than their share of citizens who are frequently described as "colorful" - Sheriff Leon Lott, who purchased an armored personnel carrier for his police force, and famously broke down a door based on a picture of gold-medalist Michael Phelps holding a bong; Mayor Sallie Peake, who banned the police from chasing suspects on foot in Wellford, SC; noted hunter Nathan Dickson; Joshua Glidewell, a Fred Phelps wannabe who's suing the city of Greenville for not allowing him to harass passersby.
They're the only state in the nation who elects their National Guard adjutant; one candidate for that office, Republican Dean Allen, recently raffled off an AK-47 at a fundraiser. (You'd think that one of his opponents would point out that Mr Allen not only couldn't even be counted on to buy American, but felt it was appropriate to raffle off a weapon from the former Soviet Union.)
South Carolina also has a long-standing militia movement - people deluded enough to believe that a hundred hillbillies with hunting rifles can overthrow the entire US government.
You should probably also remember Caitlin Upton, the Miss Teen USA contestant from South Carolina, who gave this fascinatingly incoherent response when asked why 20% of Americans can't locate the US on a world map:
I personally believe that U.S. Americans are unable to do so because, uh, some, people out there in our nation don't have maps and, uh, I believe that our, uh, education like such as, uh, South Africa and, uh, the Iraq, everywhere like such as, and, I believe that they should, our education over here in the U.S. should help the U.S., uh, or, uh, should help South Africa and should help the Iraq and the Asian countries, so we will be able to build up our future, for our (children).And she was an honor student in high school. (Admittedly, a South Carolina high school, so the bar is pretty low.)
And their politicians are notoriously unstable. As far back as 1856, Representative Preston Brooks used his walking stick to beat another congressman, Charles Sumner, so badly that Sumner couldn't return to his congressional duties for three years. (Brooks actually had a long history of violent behavior - he never finished law school because he was expelled for threatening the police with a gun. However, his violent impulses apparently only extended to attacking without provocation - after being denounced for his actions by a third Congressman, Anson Burlingame, he challenged Burlingame to a duel, but chickened out when Burlingame accepted.)
Strom Thurmond was an evil old racist from South Carolina, who kept his Senate seat for 49 years following 2 years as governor. One of his many notable remarks was that "there's not enough troops in the army to force the Southern people to break down segregation and admit the nigra race into our theaters, into our swimming pools, into our homes, and into our churches." And he still holds the record for the longest filibuster in Senate history; he talked for 24 hours and 18 minutes opposing the Civil Rights Act of 1957. (Of course, this didn't prevent him from having a daughter out of wedlock with his black maid. Go figure.)
More recently, we had South Carolina Representative Joe Wilson, who felt that shouting "You lie!" at the President of the United States was an appropriate way to uphold the dignity of his office. (Wilson was also a strong supporter of Strom Thurmond, and an advocate for the aforementioned Confederate flag, in spite of, or perhaps because of, the fact that it's a symbol of slavery and repression.)
And lest we forget, Governor Mark Sanford, recently in the news for his habit of taking publically-funded junkets
All of which brings us back to our original question: What the hell is wrong with South Carolina?
I can't answer that question, but until I'm given evidence to the contrary, I will continue to believe it's inbreeding.