This is a a very popular question. Rule of thumb is to double-space everything.
Double space throughout.This includes all text in your manuscript, titles, headings, footnotes, references, quotations (under and over 40 words), and figure captions.
Never use single-spacing, or one-and-a-half spacing except in tables, or figures.
Triple and Quadruple-Spacing
Triple and quadruple spacing is acceptable in special circumstances.
(American Psychological Association, 2010, pp. 171, 229)