"From our research, we found that stocks in Thailand offered the highest returns in terms of value, real return and real return after tax. The outcome aligned with case studies from several other countries, including the US, the UK, and Japan.

"However, the value of stocks fluctuated the most," he said.

Even though stocks yielded the biggest return over the 30-year period, there were times during this period when stock holdings offered the lowest return when compared with bonds, cash and gold. This was because the return on stocks is typically the most sensitive to economic and other factors, he said.

The research chose three periods to make up the total of 30 years: between 1975 and 1996, from 1997 to 2000, and from 2001 to 2005.

"Investors might encounter stock fluctuations in some periods. For example, stocks provided the lowest return among other kinds of investment between 1997 and 2001. Deposits at that time offered more than a 10-per-cent return, while bonds gave a 7-per-cent return," he said.

"During the pre-financial-crisis period, stocks yielded around 16 per cent per annum, higher than returns from deposits, bonds and gold, all at less than 10 per cent. Stocks returned to be the best performing investments after the crisis, with a return around 20 per cent per annum, while other assets offered constant returns."

Despite the high volatility of share prices, stock investors have a greater chance of obtaining higher returns than those who put their money into other assets - if they hold stocks over a longer period, he said.

"There is a 53-per-cent possibility stock investors will receive higher returns than bond holders if they hold on to their investments for two years," he said. "The possibility increases to 59 per cent if they hold their stocks for five years."

Meanwhile, the SET has introduced its "Total Return Index" (TRI) to serve as a benchmark for returns on stock investment.

 The new index takes into account returns from dividends, capital gains or losses and rights in share subscriptions.

"The SET index at present is based on returns only from capital gains, losses and rights in new share subscriptions. The TRI reflects a more complete picture of returns because it takes dividends into account," Sethaput said.

"At the moment, using the SET index as the only one benchmark results in greater returns from mutual funds," he said. "The TRI will serve as a standard benchmark to reveal the competence of fund managers."