A market's sector composition reflects which companies choose to list, not the full distribution of a country's economic activity. In an economy dominated by small businesses or family firms, a sector large in the economy may carry a low weight in the equity market. The reason is that those companies are not listed. Whether a company lists is itself a filter, and that filter is not consistent across countries. Historical path produces durable differences. A market's sector composition carries considerable inertia, with large companies that listed early occupying the bulk of the index for a long time.
Companies in newer industries need time to grow and list before they can change the composition. This leaves some markets' sector structures reflecting the economy of decades earlier, already diverging from current activity. The history of privatisation is another factor. Large listed companies in many markets originate from the privatisation of state enterprises: telecommunications, energy, finance, utilities. These companies are large and listed early, so they occupy high index weights. This explains why some markets' indices concentrate heavily in a few traditional sectors rather than in emerging ones. Industrial policy and the conditions for raising capital shape composition too.
An environment providing capital market support for a particular sector makes that sector more likely to grow through the market and occupy weight. A venture ecosystem and listing route for technology is one example. A market lacking such conditions struggles to reflect the same industrial activity in its equities, even where the activity exists. This difference bears directly on cross-country comparison. When comparing the valuation or return of two markets, failing to adjust for sector composition means comparing two different baskets of sectors rather than the relative pricing of two markets.
A substantial part of the European and United States valuation gap discussed earlier comes from sector composition rather than pricing, and the principle applies to every cross-country comparison. The way an index is constructed further amplifies or reduces these differences. Different weight caps, different inclusion criteria, and different treatment of particular ownership structures all change the final sector distribution. Two indices named for the same market can show noticeable differences in sector composition if they use different rules. For investors, the practical approach treats sector composition as the first step of analysis rather than the last.
A substantial part of a market's direction is set by the cycles of its major sectors. Understanding what those sectors are, how much weight they carry, and what they are sensitive to explains its behaviour better than the index name or an overall valuation figure. The same principle applies to judging a market's sensitivity to a particular external factor. Where the major weights concentrate in interest-sensitive sectors, the market's reaction to the rate environment will be stronger than that of a sector-diversified market. Sensitivity to any given driver is therefore a property of composition, and a headline index figure conceals it entirely. A market that appears defensive by valuation can behave cyclically if its weights sit in cyclical sectors, and the label attached to it will not reveal that in advance.