So reCaptcha uses a statistical accuracy and machine learning algorithms to compare the user input for the same word over hundreds of users. Thus by doing this

So reCaptcha uses a statistical accuracy and machine learning algorithms to compare the user input for the same word over hundreds of users. Thus by doing this they minimize the probability that a human who incorrectly deciphers a word skews their data. Also reCaptcha's present two words. One of which is known, and used as a normal captcha, the other of which is an unreadable word. Thus if a someone passes the captcha word, its somewhat assumed that he also correctly typed in the unknown word because the words are presented in randomized order. Finally, as a funny anecdote, Luis told me once of this time that they noticed there was a captcha farm in latin America paying pennies for poor latin american's to solve reCaptcha's for them in order to gain the ability to generate illegal spam. Luis started sending them whole sentences of reCaptcha's and eventually made it to full paragraph's. Ironically, the reCaptcha farm continued to solve these captcha's. He eventually stopped their activity, but he said the data they generated was some of the best he's ever gotten... :P