A folding passenger seat assembly for a motor vehicle comprises a seat cushion and a backrest portion. The backrest portion includes a backrest frame having a first end and a second end each pivotally supported about a folding axis. A first latch selectively locks the first end of the backrest frame in the vertical use position and a second latch selectively locks the second end of the backrest frame in the vertical use position. A remote actuator simultaneously unlocks both the first and second latches so that the backrest frame can be pivoted between use and folded positions. The second latch includes a bell-shaped jam key which is pivoted on a pintle about a latch axis spaced parallel from the folding axis. An abutment seat is fixed relative to the folding axis for engaging the jam key when the backrest frame is locked in the vertical use position. The jam key has a convex engagement surface having a circular curvature centered about the latch axis, while the abutment seat includes a concave engagement surface having a circular curvature centered about the latch axis only when the backrest frame is in the use position. The distance between the latch axis and the abutment seat diminishes as the backrest frame is moved from the use position toward the folded position. As a result, the jam key is compressed between the pintle and the abutment seat if the backrest frame is forced toward the folded position while the first and second latches are locked.