The markets churn, the world turns, yet half of all investors remain unmoved.

According to research released Monday by New York-based Dreyfus, 49 percent of 1,250 individual U.S. investors indicate that they have not taken any action to re-evaluate their portfolios, even as a bull market in equities enters its eighth year.

While most market prognosticators predict that investors will have to navigate prolonged periods of lower returns, those concerns are not filtering down to many investors, according to Dreyfus’s "Helping Meet Investor Challenges Study." The long-lasting economic recovery may be causing complacency among market participants, some of which may be attributed to behavioral inertia, a natural tendency to favor the status quo over change.

Dreyfus found that investors tend to become more implacable with age: While 61 percent of respondents aged 55 and older have no plans to re-evaluate their approach, most investors between 21 and 34, roughly corresponding to the millennial generation, are consistently reviewing their portfolios. Almost two-thirds of the millennial respondents, 65 percent, had already evaluated their approach.

Greater proportions of millennials also indicated that they had worked with an advisor to review their investments. While 63 percent of millennials had worked with an advisor, only 38 percent of respondents aged 55 and older had evaluated their portfolio with an advisor.

Respondents who had worked with an advisor were far more likely to review and adjust their portfolios. Among all the respondents working with an advisor, 65 percent said they had re-evaluated their investment approach, compared to 40 percent of the respondents who had not worked with an advisor.

Most investors without a financial advisor, 60 percent, reported that they were most likely to delay or put off their plans to address changing market conditions. Another 17 percent of those without an advisor said they didn’t have any plans to revisit their investment approach.

Among mass affluent respondents with between $250,000 and $2.5 million in AUM, 43 percent had not re-evaluated their investment approach.

For the study, Dreyfus, a subsidiary of BNY Mellon, sponsored a survey this year of 1,250 individual investors aged 21 or older with at least $50,000 in investible assets.