This an interesting study performed in 2009 by NHSTA. I find it interesting that IIHS does not point to or reference this more recent study on their FAQ in section 7.
http://www-nrd.nhtsa.dot.gov/Pubs/811208.PDF If you take the time to read through it carefully, you'll see how it can be easily "spun" to support helmet laws and the cost of healthcare. However, the data doesn't support non-helmeted riders to be a contributing factor over helmeted riders. While you can make big headlines that non-helmeted riders suffered more TBI than helmeted riders - the actual numbers between the two categories are small. A relevent excerpt as follows (I added the emphasis):
"In the data set,
57 percent of motorcyclists were helmeted at the time of the crashes and 43 percent were non-helmeted. For both groups, about 40 percent of motorcyclists were treated at hospitals or died following the crashes. However,
6.6 percent of unhelmeted motorcyclists suffered a moderate to severe head or facial injury
compared to 5.1 percent of helmeted motorcyclists. Moderate to severe injuries were defined as a Maximum Abbreviated Injury Severity (MAIS) scale of level 2 or higher
Fifteen percent of hospital-treated helmeted motorcyclists suffered traumatic brain injury (TBI) compared to 21 percent of hospital-treated unhelmeted motorcyclists. TBI severity varied by helmet use. Almost 9 percent of unhelmeted and 7 percent of helmeted hospital-treated motorcyclists received minor to moderate TBI. More than 7 percent of unhelmeted and 4.7 percent of hospital-treated helmeted motorcyclists sustained severe TBI.Median charges for hospitalized motorcyclists who survived to discharge were 13 times higher for those incurring a TBI compared to those who did not sustain a TBI ($31,979 versus $2,461). Over 85 percent of hospital-treated motorcyclists without a TBI were discharged home, compared to 56 percent of motorcyclists with severe TBI. Motorcyclists admitted to the hospital with TBI were more likely to die, be discharged to rehab, or transferred to a long-term care facility. While 17 percent of all hospital-admitted motorcyclists had TBI, they account for 54 percent of all admitted riders who did not survive." <--
Note that in this paragraph it does not distinguish between "helmeted" vs "non-helmeted" riders, ony those riders that suffered TBI.
So from the above study:
6.6% of non-helmeted riders suffered MAIS
5.1% of helmeted riders suffered MAIS
Difference = 1.5%
15% of helmeted riders suffered TBI
21% of non-helmeted riders suffered TBI
Difference = 6%
However, remember that the total percentage of helmeted riders in this study were larger, 57% to 43% - so the
actual numbers are as follows(rounded to nearest real person):
Total helmeted riders = 59,549
Total MAIS = 3,037
Total TBI = 8,932
Total non-helmeted riders = 44,923
Total MAIS = 2,965
Total TBI = 9,434
Bottom line here is that it really makes no appreciable difference whether people wear a helmet or not as far as healthcare costs go. Helmeted riders are not imune from MAIS and TBI. While I will contend that in the above study, non-helmeted riders (as a percentage of non-helmeted riders) suffered more MAIS and TBI than helmeted riders (as a percentage of helmeted riders) - the numbers do not support a "causation" difference for total healthcare costs. You have to keep in mind that once riders are classified as "with TBI" - it doesn't matter if they were helmeted or not - the costs per the study are the same for both groups. The study goes on to state:
"A logistic regression analysis that accounted for clustering of motorcyclists within States indicated that helmets significantly reduced the odds of sustaining head or facial injury, TBI, and dying in the hospital."
I don't know what a "logistic regression analysis" when applied to "clustering of motorcyclists" is, but the actual numbers state 6% difference in TBI. Not sure I see that as "significant".