Ignoring fermions in string theory/supergravity Often in string theory/supergravity, papers will say something along the lines of "we set all the fermions to zero and focus purely on the bosonic field content". See e.g. the final lines on p3 of hep-th/0605203:

To place the results in physical context, let us recall the general
  strategy of finding vacua in heterotic string theory with spacetime
  supersymmetry. Instead of solving the full fledged string equations of
  motion, which include all massive modes, one finds a supersymmetric
  configuration in the field theory approximation. In particular, one
  solves for a bosonic configuration in which all fermions can be
  consistently set to zero.

I have two questions:


*

*Why would you do this? I guess the answer is probably to simplify calculations.

*More importantly, why is this allowed? They all say that it is consistent to truncate these fields. But why? 
 A: I think this deserves a proper answer, since the answer by @JohnDoe is incorrect and should be amended. A consistent truncation is, by definition, setting to zero (or to some fixed value) some of the fields of the theory in a way consistent with its equation of motion. In other words,a consistent truncation identically satisfies the equations of motion of the truncated fields. For a supersymmetric theory, it turns out that truncating the fermions is always consistent. This can be explained by noticing that the lagrangian of such theories enjoys a well-defined $\mathbb{Z}_{2}$ symmetry on the fermions.
For supergravity theories, one truncates the fermions for example when one is interested in obtaining classical "macroscopic" solutions of these theories, since they cannot contain fermions.
A: *

*Depends on the context. Normally, people use bosonic truncation of supersymmetric actions because supersymmetry fixes relative coefficients in a Lagrangian for a given model. 

*It is not "allowed". It is never consistent to truncate all the fermionic fields. Supersymmetry means you have equal number of bosons and fermions. Once you truncate "all" fermions, then you have to truncate "all" bosons as well. However, there are cases where you can truncate some of the bosonic and fermionic fields together. Usually, these fermionic and bosonic fields form a "multiplet" and usually you can truncate a multiplet from your theory depending on what you are interested in.
