There’s good reason for differentiating sharply between “whole” and “half” steps in a musical scale: only whole steps feel like definite steps. Half steps feel iffy at best. The pentatonic scale, popular around the world, can be defined as the scale that wants nothing to do with half steps. Our standard eight-note scale contains half steps between the third and the fourth (like between E and F) and between the seventh and the first (B and C); the pentatonic scale (C-D-E-G-A) leaves out the fourth and seventh. Problem solved.
Crowd-friendly tunes like “Yankee Doodle” use the whole eight-note scale but with pentatonic bias, emphasizing the pentatonic scales corresponding to their changing chords. In “Yankee Doodle” there is indeed a half step in the second line between the syllables of [a-riding on a] “po – ny,” but “po-” is on the first note of a C pentatonic scale, the chord at that moment being C, and “-ny” is on the third note of a G scale, the chord at that moment being G. You experience the congenial chord change more than you experience the insidious half-step melodic move (but with a frisson).
However! Once “Yankee Doodle” has won your confidence with the strong pentatonicism of its first two lines, it gets cocky and throws in some half steps with no direct pentatonic justification, like “feather in his cap” and, amazingly, “Yankee Doodle, keep it up.” Of course, this isn’t amazing; half steps are as common as dust particles. They’re put in all the time to get you from one melodic point to another, perhaps to ration out the time of transit between two places, perhaps to thicken the sense of passage:
(And check out this commitment to half steps in the bridge:)
They’re also often like Secret Service agents: you are supposed to be aware of them, but you shouldn’t pay too much attention to them. They’re not really things or places to be in their own right. When a song calls attention to a half step and offers it as a thing and a place to be, there had better be a good rationale—either a purpose for the discomfort, like the suspense ratcheting in “Manic Depression,” or else some comfort in the discomfort, like being smashed while gloomy in the Jack Bruce/Pete Brown song inspired by Under the Volcano:
When he walks from the Consul at sunset
Barely remembers his name
Walk is a little unsteady, sadly
But he knows most of all that he’s living beneath the volcano
Won’t be so many more days
Notice the volcano as the emblem of death in this scene, and the musical intensity of the singer facing death by taking half steps downward (as though sleep-shuffling into the volcano) in fierce harmony with the held top note (as though consciously hanging on at the volcano’s lip, or the lip of the whiskey glass). The union of the unchanging top note with the descending notes makes the pain wonderfully worse.
Consider how different “I’m A Man” would feel if it used the same strategy in its half step hook:
Spencer Davis Group, “I’m A Man” ______________________________________________________________
 Trilling, though—oscillating rapidly between a main note and the note a half-step up or down from that—requires not taking a whole step, and is all over Middle Eastern music.
 The score of the song shows the held top note as the melody – but my experience is dominated by the descending notes. Take them out and what’s the point?
 There’s a fairly close precedent in “Shapes Of Things” by the Yardbirds (1966).
The splitting apart of the vocal line at the end of the verse gives you a victorious feeling that neither of the attractive melodic paths, downward-descending by steps or holding one note as we heard before, needs to be given up to take the other.