The sunk cost fallacy is the reason "but we've been together seven years" feels like an argument for year eight. It's the cognitive bias of continuing to invest in something because of what you've already put in — time, money, effort, emotional energy — rather than what it's actually giving you or likely to give you. As The Decision Lab defines it, it's "our tendency to follow through with something that we've already invested heavily in, even when giving up is clearly a better idea." Economists named it; couples live it.

The brutal logic underneath: sunk costs are sunk. The years are spent whether you stay or leave. Staying doesn't refund them — it just adds more.

What Does the Sunk Cost Fallacy Look Like in a Relationship?

  • Your reasons to stay are all in the past tense. "We've been through so much." "I've put so much work in." Notice none of these describe what the relationship is like now.
  • The anniversary math does the arguing. Every time you consider leaving, a number shows up — five years, the wedding cost, the apartment — and out-votes how you actually feel.
  • Leaving feels like admitting failure. Not like a decision about the future, but like a verdict on the past — as if ending it would mean it all "meant nothing."
  • You double down after bad stretches. The worse it gets, the harder you work, because quitting now would "waste" the effort already spent fixing it. That's escalation of commitment: investing more to justify having invested.
  • You'd tell a friend to leave. The classic tell. Their five years look clearly spent to you; only your own feel refundable.

Why Do Smart People Fall for It?

Psychologists Hal Arkes and Catherine Blumer, who named the sunk cost effect in 1985, showed that people reliably let unrecoverable investments steer current decisions — paying attention to what they've spent instead of what they'll get. Loss aversion does the heavy lifting: walking away converts an ambiguous situation into a certain, countable loss, and brains hate booking losses. Staying postpones the receipt.

Relationships add two accelerants. First, the investment isn't just money — it's identity. "Us" is years of self-definition, and abandoning it feels like deleting a chapter of who you are. Second, the cost is shared, so leaving means writing off someone else's investment too. None of that changes the math. It just explains why the math is so hard to look at.

In Practice

She's 34, six years in. If you ask how it's going, she exhales before answering. The affection dried up around year three; the conversations are logistics; she's cried in her car twice this month. But every time she gets close to the sentence, the slideshow starts: the early trips, the apartment they renovated, the version of him she's still waiting to come back. "I can't just throw away six years," she tells her sister. Her sister doesn't say what she's thinking — that the six years aren't the thing being thrown away. They're gone either way. The thing actually on the table is year seven, and eight, and nine, and she's about to spend them protecting an investment instead of living in a relationship.

What to Do About It

Run the fresh-start test. Knowing everything you know now, would you choose this relationship today? Answer it before the slideshow starts.

Audit the tense of your reasons. List why you're staying. Past-tense reasons ("we've built," "we've survived") are sunk costs. Present-tense ones ("I feel respected," "we want the same things") are the only ones that count.

Reframe the waste. The years happened; they held real things; an ending doesn't unhappen them. The only wasteable years are future ones.

Decide the future on the future. If the relationship is worth staying for, it will be for reasons that exist now — and those reasons are also exactly what's worth fighting to repair.

If you can't tell whether you're staying out of love or out of arithmetic, laying the reasons out with Lainie can make the tense of each one obvious.