  We argue that the shape of the dispersion along the nodal and antinodal
directions in the cuprates can be understood as a consequence of the
interaction of the electrons with collective spin excitations. In the normal
state, the dispersion displays a crossover at an energy where the decay into
spin fluctuations becomes relevant. In the superconducting state, the antinodal
dispersion is strongly affected by the spin resonance and displays an S-shape
whose magnitude scales with the resonance intensity. For nodal fermions,
relevant spin excitations do not have resonance behavior, rather they are
better characterized as a gapped continuum. As a consequence, the S-shape
becomes a kink, and superconductivity does not affect the dispersion as
strongly. Finally, we note that optical phonons typically lead to a temperature
independent S-shape, in disagreement with the observed dispersion.
