Missing Letter Effect
The missing letter effect refers to the finding that, when people are asked to consciously detect target letters while reading text, they miss more letters in frequent, function words (e.g. the letter "h" in "the") than in less frequent, content words.
Origin
Discovered by cognitive psychologist Alice F. Healy in her 1976 paper "Detection errors on the word the: Evidence for reading units larger than letters" in the Journal of Experimental Psychology: Human Perception and Performance. Healy asked subjects to circle every instance of the letter "t" in prose passages and found they missed significantly more "t"s in the common word "the" than in other words. This demonstrated that skilled readers process frequent words as whole units rather than letter-by-letter, a finding extensively replicated over subsequent decades.