Economic Drivers of Time-Varying Market Beta

# Economic interpretation of Time-varying beta (systematic risk) in portfolio analysis


# Economic interpretation of Time-varying beta (systematic risk) in portfolio analysis












What is the economic interpretation of Time-varying beta (systematic risk) in portfolio analysis and the main economic difference with costant beta. I'm not interested in how to estimate it but just the economic meaning. Thank you

## Answer by demully (score 1, accepted)

https://quant.stackexchange.com/a/53850

There's a practical answer and a theoretical one.

Looking at a time series of any stock and market, its realised beta will naturally time-vary depending on whether company-specific good or bad news happens to be released on an up versus a down day for the broader market.

Theoretically, this shouldn't happen (as a stock with more idiosyncratic noise should have an equal and opposite lower correlation to market). But it does. Also theoretically, these effects should all average out in the long-run. Maybe they indeed do. However, the period over which you measure your beta is always but one sample set, with a limited number of observations. So even if it were true that - asymptotically - betas were constant and did not time-vary, you would nevertheless expect to see them randomly fluctuate in finite samples. In other words, you can legitimately have time-varying betas that have or require precisely zero economic meaning or explanation!

Under what circumstances might there be a legitimate and intuitive reasons for betas to evolve over time? There are too many possible to try to spell them all out; but they broadly fall into two camps.

1) A change in the company, ie micro reasons. Compare and contrast the NASDAQ of the dotcom boom/bust days with the NASDAQ today. Back then, NDX/QQQ was massively higher vol and beta versus the NASDAQ of today. Their relationship to the S&P500 has clearly changed (and in quite a steady progression lasting many years). Put simply, Tech used to be super-fast revenue growth, with no profits to show for it. They were financed by pumping out tons of equity, at anyone who would take it. And whether they versus their competitors were growing at 20% versus 30% could halve or double the stock price. Now you have a bunch of cash-cow oligopolies (possibly being polite there), with sky-high RoEs, buying back their stock, and rock-solid balance sheets backstopped by those obscene unrepatriated foreign cash-piles. Tech matured; and cash-rich, mature is lower beta than a breakneck growth, low/negative margin more speculative prospect.

2) Macro, ie the things that move the market can represent more or less of what moves any stock at any point in time.

So consider eg oil companies. In a supply shock like 1973 or 1979/80, the price doubles or trebles. Good for Exxon, bad for market. Oil behaves "defensively" then (and it did). Compare and contrast to the heady days of the '00s when the Chinese were buying it faster than anyone could pump. Robust demand for oil (and for everything else to boot) was good for oil, and good for market. The Energy sector did exhibit a much higher beta back then.

You could make a similar case for eg Banks, if say a central bank were aggressively hiking to slow the economy down and kill inflation. Yes I know, ha ha, that doesn't happen anymore ;-) But that scenario would be bad for traditional cyclical industrials and the broad market, but good for the banks (at least in relative terms). As such, more from the central bank that is good for banks is bad for the market. Equals lower beta (and vice versa into aggressive dovishness moving markets).

Industrial stocks can have very different betas given a weak or strong home currency. FX affects their global competitiveness; which affects their ability to participate in any global economic pickup or their vulnerability to a global slowdown.

Basic Materials and Consumer Discretionary will likewise behave differently to any inflationary/deflationary impulse is coming more from the high street/down the mall verusus from the base metals complex. And so on.

The common thread is that whatever is moving markets at any point in time will have a stronger or weaker positive or negative impact on the most and least affected sectors and stocks. So as the source of the economic news to which markets respond time-varies, this in turn has varying impacts on those stocks' sensitivity to markets.

hope this helps.