What the viewer is actually doing in that one second

A very fast classification, on very little information. At the size a profile picture displays, expression subtleties are simply not present.

What survives to be judged

The outline of the head, two or three masses of light and dark, and whether the eyes are visible. That is the whole input.

This is why the confident advice about smiling and posture overreaches: it discusses signals that are not transmitted at the size the headshot is seen.

The defensible claim

A legible portrait prevents one specific failure — someone briefly willing to look at you scrolls past instead. It adds no competence and signals no seniority.

Modest, and it holds up.

Where the effect actually accumulates

Not in any single view. You appear in hundreds of lists over years, and a legible image in each is a small repeated advantage. Repetition rather than impact, which is a duller mechanism than the marketing describes and a more reliable one.