  We present high-resolution 3D smoothed particle hydrodynamics simulations of
the formation and evolution of protostellar discs in a turbulent molecular
cloud. Using a piecewise polytropic equation of state, we perform two sets of
simulations. In both cases we find that isolated systems undergo a
fundamentally different evolution than members of binary or multiple systems.
When formed, isolated systems must accrete mass and increase their specific
angular momentum, leading to the formation of massive, extended discs, which
undergo strong gravitational instabilities and are susceptible to disc
fragmentation. Fragments with initial masses of 5.5 M_jup, 7.4 M_jup and 12
M_jup are produced in our simulations. In binaries and small clusters, we
observe that due to competition for material from the parent core, members do
not accrete significant amounts of high specific angular momentum gas relative
to isolated systems. We find that discs in multiple systems are strongly
self-gravitating but that they are stable against fragmentation due to disc
truncation and mass profile steeping by tides, accretion of high specific
angular momentum gas by other members, and angular momentum being redirected
into members' orbits. In general, we expect disc fragmentation to be less
likely in clusters and to be more a feature of isolated systems.
