Large-cap index members disperse further

The largest companies of the S&P 500 continue to eat up market-cap share as the index climbs to new highs. During Monday's session, the S&P 500 climbed to at least 3,282.4 by midafternoon for a 2020 year-to-date return of 1.6% in the opening weeks of the new year. Amid the continuation of the rally started in October, Google moved closer to joining Apple and Microsoft as the world's select few trillion-dollar companies. The story of this new breed of megacap companies is showing that while the index is driving ever higher, only a select few are doing the heavy lifting.

The 10 largest companies currently make up 25% of the index's $29 trillion in total market cap, a figure that picked up significant momentum over the past 12 months. At the end of 2018, the top 10 companies represented about 21% of the index valuation and 20% at the end of 2016. Additionally, over the past 12 months, the standard deviation of market caps rose to $120.3 billion, $20 billion higher than a year before, and nearly double that of what it was three years earlier.