We present optical spectroscopy of 71 photometric candidate low-mass members of the cluster associated with { \sigma } Orionis .
Thirty-five of these are found to pass the lithium test and hence are confirmed as true cluster members , covering a mass range of { \leq 0.055 } - 0.3 M _ { \hbox { $ \odot$ } } , assuming a mean cluster age of { \leq 5 } Myr .
We find evidence for an age spread on the ( I , I - J ) colour magnitude diagram , members appearing to lie in the range 1-7 Myr .
There are , however , a significant fraction of candidates that are non-members , including some previously identified as members based on photometry alone .
We see some evidence that the ratio of spectroscopically confirmed members to photometric candidates decreases with brightness and mass .
This highlights the importance of spectroscopy in determining the true initial mass-function .