I'm not PG but I remember reading that it was a sliding scale based on the average karma... so if your karma grows slower than the total karma, you might have lost the ability.<p>I can't find the interesting link with the analysis I read it from (it was from around the time PG removed the points), but there's some confirmation here:<p><a href="http://jacquesmattheij.com/The+Unofficial+HN+FAQ#cantdownvote" rel="nofollow">http://jacquesmattheij.com/The+Unofficial+HN+FAQ#cantdownvot...</a>