Japan and U.S. stock rallies fueled by tech on AI boom

TOKYO -- As  Japanese and U.S. shares reach new highs fueled by the tech sector, strong earnings have allayed concerns about overheating, but the concentration of money in a handful of stocks has invited comparisons to the 1990s dot-com boom.

The Nikkei Stock Average broke the 40,000 mark for the first time Monday, following record highs set by the tech-heavy Nasdaq index in the U.S. last week. Bullishness about artificial intelligence has lifted American stocks, creating a ripple effect in Japan and elsewhere that shows no sign of abating.