One of the most basic concepts in economics is that price is a function of supply and demand. Yet, equity investors tend not to think much about shifts in the quantity they’re buying and selling.

Factors that affect intrinsic value (earnings) are endlessly debated, as is the impact of central bank asset purchases on stock prices. But rare is the market report that begins: “The price of equities rose today because there weren’t as many good ones to buy.”

Joking aside, the shrinking supply of equities - a trend known as “de-equitisation” - deserves attention, not least because it might help explain why stock prices keep touching record highs. 

Remarkably, the number of US public companies has fallen by almost half over the past two decades, even though the US economy and population grew. A decline can also be observed in the UK and Germany. Takeovers, the rise of venture capital and the growing regulatory burden of going public have all contributed to that drop, according to Credit Suisse. In addition, today’s asset-light tech start-ups simply don’t require as much capital to fund expansion.

It would be a stretch to argue that this shift in itself has pushed up stock prices, especially lately. Most of the drop in listings in the US happened during the dotcom era boom and bust. However, the stock market has also become a more profitable place for those that have survived the cull.

Thanks to rampant consolidation and laissez-faire anti-trust enforcement industries are becoming increasingly oligopolistic. Less competition makes it easier for public companies to fatten margins, and higher profits affect valuations. 

Negative

But that’s not all. Since 2009, US public companies have spent about $3trillion (R39trln) buying back stock. That constrained the supply of stock, and the price went up in part because buybacks increase earnings per share.

Yet, rising equity valuations haven’t triggered a boom in initial public offerings. Companies that have gone public find it’s much cheaper to issue debt than equity. The upshot is that net equity issuance by non-financial companies has been negative by almost $6trln over the past two decades, according to data.   To be clear, I’m not saying investors are poised to run out of stocks to buy. Rising prices simply mean you get fewer shares for your money. Also, share buybacks aren’t nearly as popular in Europe, where banks have been issuing plenty of new stock to boost capital buffers, plus China and other emerging markets are minting scores of public companies. 

However, the US accounts for a third of the world’s market capitalisation and almost 60percent of the MSCI World Index. US investors typically have a very pronounced domestic bias in their portfolios, and are turning to passive strategies such as index funds. “If you get fund flows coming into the US, you will see stocks getting bid higher because there are not enough shares,” said Jefferies strategist Steven DeSanctis.

Corporate buybacks have begun to slow and, if that continues, it could become a headwind for stocks. Until that happens, a lack of supply looks to be the market’s secret sauce.

Chris Bryant is a Bloomberg Gadfly columnist covering industrial companies. He previously worked for the Financial Times.