  Recent work has emphasised the importance of vector and tensor contributions
to the large scale microwave anisotropy fluctuations produced by cosmic
defects. In this paper we provide a general discussion of these contributions,
and how their magnitude is constrained by the fundamental assumptions of
causality, scaling, and statistical isotropy. We discuss an analytic model
which illustrates and explains how the ratios of isotropic and anisotropic
scalar, vector and tensor microwave anisotropies are determined. This provides
a check of the results from large scale numerical simulations, confirming the
numerical finding that vector and tensor modes provide substantial
contributions to the large angle anisotropies. This leads to a suppression of
the scalar normalisation and consequently of the Doppler peaks.
