How many items can the human brain hold in working memory?

George Miller's 1956 paper noted that people hold about 7 (plus or minus 2) items in immediate memory. Later work by Nelson Cowan found that when rehearsal and chunking are blocked, the real capacity is closer to 4 items. Miller himself treated the number partly as a joke about coincidences; it became one of the most cited claims in psychology anyway.

Source: Miller (1956), Psychological Review; Cowan (2001), Behavioral and Brain Sciences