When do you need to use a hyphen for compound words?

If a compound adjective can be misread, use a hyphen. 

In a temporary compound that is used as an adjective before a noun, use a hyphen if the term can be misread or if the term expresses a single thought (i.e., all words together modify the noun).

For example:

• "the adolescents resided in two parent homes" means that two homes served as residences, whereas if the adolescents resided in "two-parent homes," they each would live in a household headed by two parents.

A properly placed hyphen helps the reader understand the intended meaning.

Also use hyphens for

Compounds in which the base word is

• capitalized: pro-Freudian
• a number: post-1970
• an abbreviation: pre-UCS trial
• more than one word: non-achievement-oriented students

All "self-" compounds whether they are adjectives or nouns

• self-report
• self-esteem
• the test was self-paced

Exception: self psychology

Words that could be misunderstood

• re-pair [pair again]
• re-form [form again]
• un-ionized

Words in which the prefix ends and the base word begins with the same vowel

• meta-analysis
• anti-intellectual
• co-occur

Most compound adjective rules are applicable only when the compound adjective precedes the term it modifies. If a compound adjective follows the term, do not use a hyphen, because relationships are sufficiently clear without one.

• client-centered counseling
  but
  the counseling was client centered
• t-test results
  but
  results from t tests
• same-sex children
  but
  children of the same sex

Write most words formed with prefixes and suffixes as one word.

Prefixes

• aftereffect
• extracurricular
• multiphase
• socioeconomic

Suffixes

• agoraphobia
• wavelike
• cardiogram

When two or more compound modifiers have a common base, this base is sometimes omitted in all except the last modifier, but the hyphens are retained.

• Long- and short-term memory
• 2-, 3-, and 10-min trials

See the Publication Manual for exceptions to these principles.

(adapted from the sixth edition of the APA Publication Manual, © 2010)