Every good horror film has a final fright, usually just when you thought it was over.

If you've checked your KiwiSaver balance in the last week you may well have had that scare?

So how bad was it? Why did markets freak out in October and will they be bouncing back?

The slump through October was actually the worst sell-off the world had seen in seven years, says Pie Funds chief executive Mike Taylor.

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Wall Street's S&P-500 ended off 6.8 per cent and the NZX-50 was down 6.4 per cent for the month.

At one point both were off nearly 10 per cent from their last peak.

That's called "correction" territory in market jargon.

"So for many people their KiwiSaver accounts will have gone down quite a bit and it will have been quite a shock because we haven't seen that for a long time," Taylor said.

The logical explanation for the slump was rising US interest rates.

"It's pretty similar to what we talked about earlier in the year [the February sell-off], he said.

"The US has a lot of highly valued technology stock that have instituted buy-backs over the years and that's all fueled by cheap money and credit. So when credit gets more expensive the valuation of those companies just changes."

While US interest rates have been rising steadily for two years, there was a spike in late September which was something of a trigger.

But for all that there is something more than superstition to the so called "Halloween effect" with a statistically unusual number of crashes, corrections and sell-offs in October, Taylor said.

Humans look for pattern recognition, and there was possibly a self-fulfilling element to the superstition.

The old traders adage: "sell in May and go away" did actually some hard data to back it up, he said.

The research shows that if you are in the market between May 1 and October 31 your performance is almost five per cent worse than if you are in from November April.

"Most of the trading happens in the Northern hemisphere where people have come back from holiday and reassessed there portfolio in September," he said. "That often means that October is the month when most of the sell decisions get made."

Despite the markets already bouncing back this month, it was unlikely a case of normal service resuming as long as interest rates continued to rise.

There was possibility that a resolution on the US/China trade war tensions might further boost markets, or if the rate rises were paused there may be a return to strong growth for a while, Taylor said.

But the US interest rates remain the real risk.

"The Fed, or interest rate hikes, have killed off most of the bull markets in the last 30 or 40 years. That is the danger. If they keep rising interest rates they will eventually kill-off the bull market. Because they just turn off the tap."