Which effect makes a clock lower in a gravitational field tick more slowly than one higher up?

The effect that makes a lower clock tick more slowly than a higher clock is gravitational time dilation.

General relativity treats gravity as a feature of spacetime geometry rather than simply a force. A clock deeper in a gravitational field, or at lower gravitational potential, accumulates less proper time than a clock farther away. The difference is tiny near Earth but becomes substantial near very massive or compact objects.

Albert Einstein described the effect in 1907 using the equivalence principle, which links gravity with accelerated reference frames. The first direct laboratory confirmation came from the Pound–Rebka experiment in 1959, which measured the associated gravitational redshift. Modern optical clocks can detect height differences of less than a metre.

This is not the same as ordinary motion-based time dilation, which results from relative speed. Both effects matter in satellite navigation: GPS clocks experience gravitational and velocity-related shifts and must be corrected. Gravitational redshift is closely related, because light emitted by a lower clock is observed at a lower frequency by an observer higher in the field.