The Collaborative International Dictionary of English v.0.48:

Malthusianism \Mal*thu"sian*ism\, n.
   The system of Malthusian doctrines relating to population.
   See Malthusian.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Malthusianism
    n 1: Malthus' theory that population increase would outpace
         increases in the means of subsistence [syn:
         Malthusianism, Malthusian theory]