Nortel and the Hidden Risk Inside Capitalization Weighting
At its peak the Canadian telecom equipment maker represented roughly a third of the value of Canada's benchmark index. Its collapse is the strongest argument against concentrated index exposure.
The Concentration
During the telecom boom around 2000, Nortel Networks grew to represent roughly a third of the total value of Canada's benchmark stock index.
That figure is worth pausing on. An investor holding a Canadian index fund, believing themselves diversified across the country's largest companies, held approximately a third of their equity exposure in one telecommunications equipment manufacturer.
The company subsequently collapsed, filing for bankruptcy protection in 2009, and shareholders were effectively wiped out.
How Index Weighting Produces This
Most indices weight constituents by market capitalization, so a company's weight rises as its price rises. That has a specific consequence that is easy to state and hard to internalize.
The mechanism means an index fund automatically holds more of whatever has risen most and less of what has fallen. In a bubble concentrated in one sector, index investors accumulate ever larger positions in exactly the securities becoming most overvalued.
Capitalization weighting is not neutral. It systematically increases exposure to whatever has appreciated most, which is the definition of buying high.
Why Smaller Markets Are More Exposed
The effect is most severe in smaller national markets. The United States index contains many large companies across varied sectors, so even a very large constituent represents a modest share.
A smaller market may have a handful of dominant companies, so a single successful firm can grow to a weight that would be impossible in a larger index. Several national indices have experienced comparable concentration, and investors in those markets carry single company risk while believing they hold a diversified national portfolio.
The Pension Dimension
The consequence was not confined to individual investors. Canadian pension funds and retirement accounts tracking the index held the same concentrated exposure, so the collapse affected retirement savings broadly.
This is what makes the case more than a curiosity. Index investing is generally excellent advice, and it carries a specific structural risk that is invisible until concentration becomes extreme.
What to Actually Do
Three practical responses follow. Check the top holdings and their combined weight in any index fund held, which takes a minute and is rarely done. Consider equal weighted alternatives, which cap the influence of any single constituent, accepting different tradeoffs including higher turnover.
And diversify across geographies, since concentration in one national index is diluted by holding others.
The contemporary relevance is direct. The largest American technology companies now represent a substantial share of the S&P 500, and the same mechanism applies, at lower concentration than Nortel reached but higher than the historical norm.
The Bottom Line
Nortel gave Canadian index investors a third of their equity in one company without any of them choosing it. Capitalization weighting concentrates into whatever has risen, so check the top holdings of anything you own.