As we approach the 90th anniversary of the Great Stockmarket Crash, could history repeat itself?
Across the two days of 28 and 29 October 1929, stockmarkets suffered the worst decline ever recorded.
The US stockmarket would not return to its peak closing level of 3 September 1929 until 23 November 1954. Compared with 1929, the world economy is much better regulated today - particularly in the wake of the Global Financial Crisis (GFC).
Nevertheless, there are some uncanny similarities.
Debt - then and now
One of the causes of the 1929 crash was the huge amount of debt in the system. Encouraged by cheap money and a sense that the good times - the Roaring Twenties - were here to stay, many people were heavily indebted. In particular, investors often bought on margin - borrowing money from a broker to purchase shares.
This phenomenon exacerbated the falls on Wall Street; widespread indebtedness left many facing ruin when the paper value of their securities turned out to provide no security at all.
Debt is still an issue today. The total global debt burden now is significantly higher than it was before the GFC.
This explains why the International Monetary Fund (IMF) and Bank for International Settlements (BIS) have expressed concerns in report after report. It also helps explain why central banks have proven to be so sensitive to signs of slowing growth.
Who owes most?
Where do the largest problems lie? Unsurprisingly, China is a major concern. Total debt officially equates to more than 250% of GDP, much of it concentrated in the housing market.
On top of this, there are sizeable off-balance sheet items and the liabilities of the non-banking sector to take into account.
For the time being, the government is using its control of the banking system to direct capital flows and prevent any individual debt servicing problems from snowballing.
A trigger to watch would be more signs that capital is trying to leave China - excessive moves could feed through into sharp currency pressures or a serious slowdown in domestic activity, with knock-on effects in emerging markets.
Europe's banking burden
A second risk is European banks' massive exposure to government debt. Again, there are some similarities to 1929.
In the aftermath of the Great Stockmarket Crash, thousands of US banks failed when borrowers proved unable to service their debts.
Today's problem was recognised during the eurozone crisis of 2012 and European Central Bank (ECB) President Mario Draghi's famous quote that the ECB is ready to do "whatever it takes" to preserve the euro.
Sadly not much has improved since then; banks' holdings of domestic sovereign bonds are little changed since 2012.
Particular questions continue to be asked about many banks in Italy and Germany - especially their inability to create strong profit growth in a world of negative interest rates.
US corporate debt
The IMF recently expressed concerns about a third area, namely the build-up of corporate debt, especially in the US.
In many cases, there has been financial engineering of the balance sheet - borrowing at low rates to finance share buybacks.
The IMF warned that 40% of all corporate debt in major economies could be considered "at risk" in another major global downturn, exceeding the levels seen between 2008 and 2009.
Global trade under strain
Today's trade tensions and rising protectionism also provoke troubling parallels with the past. In the aftermath of the 1929 crash, trade wars - accelerated by the US's Smoot-Hawley Tariff Act of 1930 - exacerbated the economic impact and contributed to the Great Depression.
Today, the rules-based global order is under serious pressure; we have seen tensions between the US and China, Canada and Mexico, potentially the US and Europe too, while Japan and Korea have begun their own tussle over trade.
'History doesn't repeat, but it often rhymes'
Whether or not Mark Twain did make the above remark, there are differences as well as parallels to the events of 90 years ago - which should mean it is different this time.
Central banks are very aware of the risks, hence the creation of new monetary policy tools and a recognition that the availability of money matters just as much as the cost of money.
Improved macro-prudential regulations have strengthened the resilience of the financial system, especially moderating credit growth to the consumer sector.
There is a growing recognition that the baton may be passed to fiscal policy to provide more support for major economies.
All in all, lessons learned mean a repeat of 1929, or indeed 2008, remains unlikely. We can never say never of course, because ultimately financial markets are built on human emotions and are prone to a sudden loss of confidence.
This makes political decision making, and a lack of policy errors, ever more important.
Andrew Milligan is head of global strategy at Aberdeen Standard Investments