NEW YORK • The initial public offering (IPO) market is manic. Stocks have not been this expensive since the dot.com era. The Nasdaq 100 has doubled in two years, leaving its valuation bloated - all while volatility remains stubbornly high. It is a set-up that has left investors sitting on fat returns from 2020, a year that defied easy explanation. It is also one that has a growing cohort of experts warning about a bubble. Knowing when market rallies turn from logical to excessive is always tough. It was nearly impossible as last year ended, with interest rates pinned near zero and the federal government unleashing another US$900 billion (S$1.19 trillion) into the economy. But history offers clues, and a raft of current market conditions meet criteria that would likely be found on a bubble checklist. Take a study by Harvard University researchers published in 2019. It noted that while not every stock surge meets with disaster, those that do share some attributes, including increased share issuance, heightened volatility, and a sector or index that doubles and is twice as high as the broader market. Check, check and almost check. "Are there areas of the market that are in a bubble? Y...