tribool question

Hi, I'm using boost::logic::tribool for expressing logical flags that may be nulls by their nature (patient not asked, he/she doesn't know etc.). Default initialization to 'false' seems to be quite unnatural in this context and initialization to 'undetermined' seems to be prefered. Is any significant reason for boost::logic::tribool default constructor initializing the value to 'false'? TIA -- Serge