It’s been 8 months since news broke of the US population reaching the 300 million mark, only 39 years after the population reached 200 million in 1967. At the time many news reports claimed that the US growth rate was accelerating. The reason? It had taken 52 years for the US population to double from 100 to 200 million, but only 39 years to go from 200 to 300 million. Hence the accelerated growth rate.
This is totally incorrect reasoning!
Using the Rule of 70 (Growth Rate x Doubling Time ≈ 70),; a 52-year doubling time implies a growth rate of approximately 1.3%. However, the 39-year-span in going from 200 to 300 million is not a doubling time - this is only an increase of 50%. Therefore, the growth rate over the last 39 years (since 1967) is approximately 1.04%, i.e. the US growth rate has declined.
So a lot of media totally botched the message - not unusual given the rampant innumeracy (a wonderful word, and terribly debilitating problem for citizenry as described so well by John Allen Paulos in his book of the same title) in this country.
Is the growth rate still diminishing? This is probably a good bet. Population growth rates in the US dropped for most of the 20 century (see the Historical National Population Estimates for a listing of yearly grown rates form 1900-1999), starting at a value of approximately 2% in 1900. (Not surprisingly, there was a slight blip upwards post-World War II)
But there is every evidence to suggest that innumeracy, at least among the media, is staying level - and maybe growing.