Much more restrictive than the previous one, the current legislation became effective after the promulgation of amendments to the 1976 Copyright Act, dated October 27th, 1998. As explained by Michael Hart in July 1999: "Nothing will expire for another 20 years. We used to have to wait 75 years. Now it is 95 years. And it was 28 years (+ a possible 28 year extension, only on request) before that, and 14 years (+ a possible 14 year extension) before that. So, as you can see, this is a serious degrading of the public domain, as a matter of continuing policy."
The dates mentioned by Michael are: a) 1790, date of the stranglehold of the Stationers' Guild (the publishers of the time) on the Gutenberg printing press (hence the 14-year copyright); b) 1909, date of the copyright reinforcement to counter the re-publishing of large collections of the public domain by reprint houses using steam and electric presses (hence the 28-year copyright); c) 1976, date of a new tightening of the copyright following the introduction of the Xerox photocopying machine (hence the 50-year copyright after the author's life); d) 1998, date of a further tightening of the copyright following the development of the internet (hence the 70-year copyright after the author's life). These are only the main lines. The Copyright Act has been amended 11 times in the last 40 years.
As stated by Tom W. Bell in Trend of Maximum U.S. General Copyright Term (with a very useful chart): "The first federal copyright legislation, the 1790 Copyright Act, set the maximum term at fourteen years plus a renewal term of fourteen years. The 1831 Copyright Act doubled the initial term and retained the conditional renewal term, allowing a total of up to forty-two years of protection. Lawmakers doubled the renewal term in 1909, letting copyrights run for up to fifty-six years. The interim renewal acts of 1962 through 1974 ensured that the copyright in any work in its second term as of September 19, 1962, would not expire before Dec. 31, 1976. The 1976 Copyright Act changed the measure of the default copyright term to life of the author plus fifty years. Recent amendments to the Copyright Act [the ones in 1998] expanded the term yet again, letting it run for the life of the author plus seventy years."