What is the source for preferred spelling in APA Style®?
The APA Publication Manual recommends Merriam-Webster's Collegiate Dictionary (2005) as the standard spelling reference for APA journals and books for nonpsychological terms; spelling of psychological terms should conform to the APA Dictionary of Psychology (VandenBos, 2007).
If a word is not in Webster's Collegiate, try consulting the more comprehensive Webster's Third New International Dictionary (2002).
If the dictionary gives a choice, use the first spelling listed; for example, use aging and canceled rather than ageing and cancelled.
(adapted from the sixth edition of the APA Publication Manual, © 2010)