Wikipedia is not a credible source (for the reasons ZNFYRH gave, as well as: just about anyone can edit it; the Wikipedia staff does not do a very good job of ensuring the articles are entirely unbiased; and Wikipedia articles do not always - or even usually - provide actual cites for their information) and certainly not valid for serious research. Since you're doing a study guide for psychology, maybe you should look to the American Psychological Association (and its counterpart organizations in other countries). Here's the link to their website:
American Psychological Association.