Mr Swanson, who joined MFS in 1985, admits you can't time cycles, but looking for signs can "give you a sense of how far you are from the waterfall". He pointed to four signals that have put him on higher alert:
One is that a third of all companies in the Russell 2000 index "don't actually make money". "A lot of that is because of disruption and tech companies, but it is a bit disturbing. This is late cycle behaviour, and if you cut corporate taxes it doesn't help companies that aren't making money."
Another indicator is a run up in margin lending by US share market investors to levels not seen since 1928. "People seem to have forgotten that stocks can go down but one day they will get a polite phone call to remind you they can."
The third sign is the increased proportion of investment industry professionals who have "not lived through the business cycle", which is perhaps a sign of how long this cycle has run for, and how potentially ill-equipped investors are to respond when it does turn.
Finally, there are signs that economic data is weakening. Global trade is weakening and manufacturing indices in large economies such as France and Germany are turning down. Services sector data in the US is also softening – something that he says "should not be happening in a robust economy".