Department of Homeland Security REAL ID Final Regulations
http://www.nctequality.org/PDFs/RealID_March2008.pdf
03/17/2008
"The government has kept secret exactly what occurs during secondary
screening, but it is generally understood to be a more invasive search than going through the standard screening airport screening line. Transgender people may be uncomfortable with the extra scrutiny that secondary screening entails and may prefer to show ID over submitting to inspection of this detail."