  The steepening (break) of the power-law fall-off observed in the optical
emission of some GRB afterglows at epoch ~1 day is often attributed to a
collimated outflow (jet), undergoing lateral spreading. Wider opening GRB
ejecta with a non-uniform energy angular distribution (structured outflows) or
the cessation of energy injection in the afterglow can also yield light-curve
breaks.
  We determine the optical and X-ray light-curve decay indices and spectral
energy distribution slopes for 10 GRB afterglows with optical light-curve
breaks (980519, 990123, 990510, 991216, 000301, 000926, 010222, 011211, 020813,
030226), and use these properties to test the above models for light-curve
steepening. It is found that the optical breaks of six of these afterglows can
be accommodated by either energy injection or by structured outflows. In the
refreshed shock model, a wind-like stratification of the circumburst medium (as
expected for massive stars as GRB progenitors) is slightly favoured. A
spreading jet interacting with a homogeneous circumburst medium is required by
the afterglows 990510, 000301, 011211, and 030226. The optical pre- and
post-break decays of these four afterglows are incompatible with a wind-like
medium.
  The current sample of 10 afterglows with breaks suggests that the
distribution of the break magnitude (defined as the increase of the afterglow
decay exponent) is bimodal, with a gap at 1. If true, this bimodality favours
the structured outflow model, while the gap location indicates a homogeneous
circumburst environment.
