November-December 2017: Bitcoin is experiencing rapid growth, and the supply of Tether more than doubles. January of 2018: BTC's price is going down, while the supply of Tether is still increasing rapidly.
It seems the correlation has more to do with the growth rate than with the supply. In moments with stagnating or slowly increasing Tether supply, the markets were bearish.
This may be related to the fact that for the Tether issuers, it doesn't make sense to react too fast to the markets' demand for TetherUSD, because if it's only a short-term fluctuation they would have to create Tethers again. So they will always lag a bit behind the real demand evolution. Also, the markets, in strong downward movements also need Tethers due to panic sells at the Tether-listing exchanges.
There are two theories to consider when trying to answer the OP's question about the correlation:
- First, there is the "conspiracy theory", that Tether is created mainly to speculate and to influence the BTC/crypto prices to take advantage of it. It would be part of a pump-and-dump operation.
- Second, there is the "fiat circulation" theory pointed out by 1Referee (Tether as a measure for fiat demand in the market).
Both theories
could be supported by the observation that TetherUSD growth seems to be related to the price curve.
However, there is a fact that seems to support the (less problematic) "fiat circulation theory": the evolution of Tether trading volume. It is today much higher than in the 2017 bubble - at least ten-fold, while the Tether supply only doubles the end-of-2017 figure. That is an indication that Tether's supply increase is due to real usage as a trading vehicle, which also corresponds to the observation that the number of exchanges listing Tether has increased.