Its negative health effects are equal to smoking 15 cigarettes a day, being an alcoholic, or never exercising. And it is twice as bad for retirees as obesity. But almost no one talks about this silent killer in retirement. What is it?

In a word: loneliness.

elderly man sitting alone at large table in gray house and eating from a bowl

Image source: Getty Images.

Before rolling your eyes, hear me out. In 2013, Merrill Lynch and Age Wave (MLAW) conducted a study asking pre-retirees what they would "miss most about work when they retire." By an enormous margin, "a reliable income" was the most popular answer.

Chart showing what pre-retirees think they will miss most about work when they retire

Image source: Merrill Lynch/Age Wave.

Then the researchers followed up by asking a group of currently retired folks what they missed the most about work. While "reliable income" was still high on the list, the responses were otherwise starkly different.

Chart showing what retirees miss most about work

Image source: Merrill Lynch/Age Wave.

If we combine "social connections," "having purpose," and "mental stimulation," these softer variables account for 65% of the most regrettable losses retirees face in their post-work lives. That tells us there's a glaring contrast between what we think is important in retirement while we're still working and what's actually important once we reach that stage.

What does this mean for you personally? Learn about the "New Social Security" on the next page.

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