This graph shows the effect of the term extensions on the growth of a model public domain.  The model is designated the "constant authorship per capita per year model."
 
 





DESCRIPTION OF THE CONSTANT AUTHORSHIP PER CAPITA PER YEAR MODEL:

(1)  The public domain consists of works on which the copyright has expired.

(2)  The model assumes that the increment (in arbitrary units) of copyrighted works created in a given year, beginning with the year 1790, is proportional to the population:

    n(year) = p(year)*.0059697e-06

where p(year) is the U.S. population for the year.
 

(3)  The population for years between census-years is computed by linear interpolation.

(4)  Absent term extensions, all copyrights expire after exactly 28 years.  (In the model, there is no holdover to
the first of the following year.)

(5)  In this model, the term extension of 1831 became effective on January 1st, 1831 (this differs from the historical case), and delayed the expiration of copyright in works created in 1803 and subsequent years by 14 years from the date these copyrights would otherwise have expired.  That is, when extensions are included in the model there are no copyright expirations from January 1st, 1831 through December 31st, 1844.

(6)  The term extension of 1909 became effective on January 1st, 1909 in this model (unlike in the historical world, where it became effective on July 1st, 1909) and delayed the expiration of copyright in works created in 1867 and subsequent years from the date these copyrights would otherwise have expired.  That is, when extensions are included in the model there are no copyright expirations from January 1st, 1909 through December 31st, 1922.

(7)  The term extension of 1976 became effective on January 1st, 1962, and delayed the expiration of copyright in works created in 1906 and subsequent years by 19 years from the date these copyrights would otherwise have expired.  That is, when extensions are included in this model there are no copyright expirations from January 1st, 1962 through December 31st, 1980.

(8) The term extension of 1998 became effective on January 1st, 1998, and delayed the expiration of works created in 1923 and subsequent years by 20 years from the date these copyrights would otherwise have expired.  That is, when extensions are included in the model then there are no copyright expirations from January 1st, 1998 until December 31st, 2017.
 

RESULT:  In this model, the size of the public domain when the copyright term is extended as described in (5) through (8) is 30.7 arbitrary units on January 1st, 2018, compared to a public domain absent term extensions of 100 arbitrary units.