Tech-heavy stock market relies on a small handful of mega stocks.

The current rally of the S&P 500 index is being led by tech-related stocks, whose weighting in the index is at a historically high level, causing concern about the durability of the bounce. The top 10 stocks hold a 29% weight in the index, and are responsible for around 70% of year-to-date performance. However, a high level of concentration at the top of the market doesn't imply weaker future returns, according to a chart from Goldman Sachs. The current crop of tech giants largely produce real earnings and strong cash flow, and concentrated leadership is far from unusual in a market-cap weighted index like the S&P 500.