http://en.wikipedia.org/wiki/Chicken#Growth_hormones

[quote]Growth hormones
Chickens grow much more rapidly than they once did and some consumers have concluded that this rapid growth is due to the use of hormones in these animals. Some consumers believe that the increasingly earlier onset of puberty in humans is the result of the liberal use of such hormones. However, hormone use in poultry production is illegal in the United States.[35] Similarly, no chicken meat for sale in Australia is fed hormones.[36] Furthermore, several scientific studies have documented the fact that chickens grow rapidly because
they are bred to do so.[37][38] A small producer of natural and organic chickens confirmed this assumption:
“If this were 1948, you might have something to worry about. Using hormones to boost egg production was a brief fad in the Forties, but was abandoned because it didn't work. Using hormones to produce soft-meated roasters was used to some extent in the Forties and Fifties, but the increased growth rates of broilers made the practice irrelevant--the broilers got as big as
anyone wanted them to get when they were still young enough to be soft-meated without chemicals.
The only hormone that was ever used in any quantity on poultry (DES) was banned in 1959, after everyone but a few die-hard farmers had given them up as a silly idea. Hormones are now illegal in poultry and eggs. The people
who advertise "No hormones" are either woefully ignorant or are indulging in cynical fear-mongering, maybe both.[5]
[quote]