There are lots of screwball factors that can skew the length of time a coin
is in circulation. A few points:
 
When Hagerty did his study, in 1971, American coinage had just gone from
silver to the sandwich. Virtually all the silver coins (1964 and prior)
were snapped up. Then the Hunt brothers did a number on the silver market
and coins began to outrun their face value.
 
So any count that includes the formerly silver coins will be skewed for
decades after 1965.
 
If any of the coins are silver dollars, the count will be skewed because we
have not minted them for a long time.
 
During times of crisis, people have tended to hoard precious metals, which
includes coinage. Even though it wasn't strictly legal, I suspect that many
Americans put gold pieces under their pillows when America went off the
gold standard. I know my grandmother did, and I still have one of the
coins.
 
Illiterate and pre-literate people don't necessarily handle coinage the
same way as "sophisticated" users might. We dug a site with four copper
coins, two of which were perfectly smooth. There is reason to believe that
jettons were sometimes circulated in a limited way, even though they were
not strictly speaking coins.
 
After America adopted its own coinage, we took a half-century or more to
shake out the Spanish and English coins. It was just a matter of supply and
demand; coins were in short supply, and you used what you could find.
 
 
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