  We present a study of close white dwarf and M dwarf (WD+dM) binary systems
and examine the effect that a close companion has on the magnetic field
generation in M dwarfs. We use a base sample of 1602 white dwarf -- main
sequence binaries from Rebassa et al. to develop a set of color cuts in GALEX,
SDSS, UKIDSS, and 2MASS color space to construct a sample of 1756 WD+dM
high-quality pairs from the SDSS DR8 spectroscopic database. We separate the
individual WD and dM from each spectrum using an iterative technique that
compares the WD and dM components to best-fit templates. Using the absolute
height above the Galactic plane as a proxy for age, and the H{\alpha} emission
line as an indicator for magnetic activity, we investigate the age-activity
relation for our sample for spectral types \leqM7. Our results show that
early-type M dwarfs (\leqM4) in close binary systems are more likely to be
active and have longer activity lifetimes compared to their field counterparts.
However, at a spectral type of M5 (just past the onset of full convection in M
dwarfs), the activity fraction and lifetimes of WD+dM binary systems becomes
more comparable to that of the field M dwarfs. One of the implications of
having a close binary companion is presumed to be increased stellar rotation
through disk-disruption, tidal effects, or angular momentum exchange. Thus, we
interpret the similarity in activity behavior between late-type dMs in WD+dM
pairs and late-type field dMs to be due to a decrease in sensitivity in close
binary companions (or stellar rotation), which has implications for the nature
of magnetic activity in fully-convective stars. (Abridged)
