Omission bias

Omission bias is the tendency to prefer inaction over action, even when action would produce better outcomes. You hold a losing investment too long rather than selling (inaction). You fail to rebalance a drifting portfolio (inaction). You do not act on a clearly superior investment opportunity (inaction). The harm from inaction feels less “your fault” than harm from action, so inaction is preferred.

Why omission bias happens

Regret aversion for action. If you do nothing and things go badly, you have no regret (“it was not my action that caused it”). If you act and things go badly, you regret the action (“I caused this”). This asymmetry drives omission bias.

Responsibility avoidance. By doing nothing, you avoid responsibility for bad outcomes. If a stock crashes and you held it (did nothing), you feel less responsible than if you sold it before the crash (an action that might have been wrong).

Status quo preference. The current state feels safe. Taking action to change it feels risky, even if the new state is objectively better.

Omission bias in investing

Holding losing stocks. A stock declines 30%. You should consider selling if fundamentals have deteriorated. But omission bias makes you hold. Selling is an action with potential regret (“it could have recovered”). Holding is inaction with no regret (“I did not cause the further decline”).

Avoiding rebalancing. Your allocation drifted from 50/50 to 60/40 stocks/bonds. You should rebalance. But omission bias makes you avoid the action. “If I sell stocks and they then soar, I will regret the sale.” Holding is safer (no action regret).

Missing opportunities. A good investment opportunity appears. But omission bias makes you avoid taking the action to invest. “If I buy and it falls, I will regret the purchase.” Doing nothing feels safer.

Avoiding allocation changes. Your 30-year-old allocation is no longer appropriate at age 50. You should increase bonds. But omission bias makes you avoid the action, keeping the old allocation.

Omission bias and sunk-cost fallacy

Omission bias and sunk-cost fallacy often co-occur. You bought a stock at $100 and it fell to $50. The sunk-cost fallacy makes you reluctant to “realize” the loss. Omission bias makes you avoid taking the action to sell. Together, they lock you into the losing position.

Omission bias and responsibility

A key driver of omission bias is the desire to avoid responsibility. An investor who did nothing feels less responsible for losses than an investor who acted. Even if both had the same outcome, the action-taker feels worse.

This is a form of self-deception. Inaction that leads to a bad outcome is just as much your responsibility as action that leads to a bad outcome. But the psychology makes inaction feel safer.

Omission bias vs. action bias

Action bias is taking action when inaction is better. Omission bias is the reverse. They can both occur in the same investor in different contexts:

• Omission bias in concentrated positions: you do not sell a losing stock (inaction).
• Action bias in trading: you rebalance unnecessarily (action).

Both are costly.

Defenses against omission bias

• Pre-commit to actions. Decide before the fact: “If the stock falls 25%, I will sell.” This removes the omission bias decision-making when the stock does fall.
• Separate responsibility from outcome. Recognize that a decision (action or inaction) is judged by the process, not the outcome. A good decision can have a bad outcome. Do not let outcome bias drive your choices.
• Use a rule for rebalancing. “Rebalance when allocation drifts 5%” is a rule that forces action against omission bias.
• Set and follow an asset allocation. An allocation rule forces you to act (rebalance) when needed, overcoming omission bias.
• Get a second opinion. An advisor or colleague can push you toward necessary action when omission bias is driving inaction.
• Track cost of inaction. A losing stock you held for too long cost you X. A drifting allocation that you did not rebalance cost you Y. Track these costs. Over time, you will see that omission bias is expensive.

See also