Round-up investing features like the one in Acorns look harmless but may quietly work against savers.

• Rounding up a purchase and investing the spare change ties investing to spending, training you to spend more.
• The small amounts are too tiny to matter, so the habit feels productive while doing little.
• Skipping the purchase entirely and investing the full amount builds wealth far faster.
• Investing in a brand just because you bought from it is its own trap — Starbucks stock, for example, got hurt by a costly push into China.

Outlook: Expect more pushback on spare-change apps as savers look for habits that cut spending instead of dressing it up as investing.