What the data actually shows
Researchers who study forgiveness — notably Everett Worthington and Robert Enright — generally describe an unforgiving state as a blend of resentment, bitterness, hostility, and the motivation to avoid or retaliate against the offender. Holding a grudge, in this framing, is not a single feeling but a maintained stance, and maintaining it takes ongoing mental and physiological effort.
The engine that keeps it running is rumination. Across studies, people who ruminate more about a transgression tend to stay angrier and find forgiveness harder; rumination repeatedly reactivates the grievance, which keeps the associated stress response switched on. Some experimental work finds that simply recalling a grudge, versus imagining forgiveness, is accompanied by higher arousal and more negative emotion in the moment.
There is reasonably consistent evidence that forgiveness — defined as reducing one's own resentment and ill will, not as excusing or reconciling — is associated with better psychological wellbeing and lower stress, and structured forgiveness interventions show modest positive effects in trials. The evidence on direct physical-health outcomes is thinner and more mixed, so the honest summary is: fairly solid on emotional wellbeing, suggestive but not conclusive on the body.
Why this feels different from how it actually is
A grudge feels like it is doing something to the other person, when in practice they are often unaware of it or unaffected. The replaying happens entirely inside you, so the cost lands on you while feeling like it lands on them. This is why the wry observation that resentment is like drinking poison and waiting for the other person to suffer keeps resonating — the mechanism really is lopsided.
Letting go also feels morally risky. People often experience forgiveness as conceding that the wrong wasn't serious, that the person has gotten away with it, or that they themselves were foolish to be hurt. So the grudge can feel like the only thing still honouring the injustice — which makes releasing it feel like a second loss rather than a relief.
And grudges are sticky for the same reason bad events generally are: negative experiences tend to be felt more strongly and remembered more readily than comparable positive ones. An injury lodges and gets rehearsed in a way that ordinary kindnesses rarely do, so the offence keeps a privileged place in attention long after it might otherwise fade.
The grudge runs inside you; the cost lands on you while feeling like it lands on them.
What the research says to do about it
The interventions with the most support treat forgiveness as a deliberate process aimed at reducing your own resentment, not as a feeling you wait to arrive. Enright's process model and Worthington's REACH model both walk people through structured steps — acknowledging the hurt honestly, building some empathy or perspective, and committing to release the ill will — and trials of these programmes show modest but real reductions in anger and improvements in wellbeing.
Interrupting rumination is one of the most practical levers, because rumination is what keeps the grievance alive. Strategies that reduce repetitive rehearsal — redirecting attention, structured writing that builds meaning rather than recirculates the grievance, or distancing techniques that view the event from a third-person perspective — tend to lower the emotional charge over time.
It also helps to separate the decisions the grudge has fused together. Forgiveness (releasing your own resentment) is distinct from condoning (saying it was acceptable), from trusting again, and from reconciling (restoring the relationship). You can forgive without reconciling, and you can keep a boundary while letting go of the bitterness. Naming those as separate choices often removes the reason people felt they couldn't let go at all.
What the research says does not help
Venting the anger repeatedly to feel better does not reliably discharge it; the research on "catharsis" generally finds that rehearsing and expressing anger tends to maintain or amplify it rather than drain it. Re-telling the grievance to keep the case alive is closer to rumination than to resolution.
Suppressing the feeling — telling yourself you've moved on while the resentment is still active underneath — also does not work well. Forced or premature forgiveness, declared before the hurt has been acknowledged, tends to be brittle and can leave the resentment intact beneath a calm surface.
Waiting for an apology or for the other person to "deserve" forgiveness leaves the outcome in someone else's hands, often indefinitely. Because the ongoing cost falls mainly on the holder, tying your relief to the offender's behaviour can keep you in the unforgiving state long after it serves any purpose. Forgiveness in the research sense is something you can do for your own sake, independent of whether the other person ever changes.
You can forgive someone you never speak to again — forgiveness and reconciliation are separate choices.
What this looks like in real life
The grudge the other person forgot
You've replayed a slight from a colleague for months; they've likely forgotten it entirely. The replaying happens inside you, keeping the stress response switched on while the person it's aimed at goes about their day unaffected. That lopsidedness is why the studies frame letting go primarily as something done for your own benefit, not as a favour to the other person.
Forgiving without reconciling
You can release the resentment toward someone who hurt you and still choose never to let them back in. Forgiveness in the research sense — reducing your own ill will — is a separate decision from trusting again or restoring the relationship. Naming those as distinct choices often removes the reason people felt they couldn't let go at all: keeping a firm boundary is fully compatible with putting down the bitterness.
Real numbers in context
This is a field built more on validated psychological scales and clinical trials than on tidy population statistics, so be wary of anyone quoting a precise figure for "how much healthier forgivers are." The dependable findings are directional: more rumination tracks with more sustained anger and harder forgiveness, and forgiveness interventions produce modest average improvements in emotional wellbeing across trials, with results varying by person and by how deep the injury was.
On physical health, claims are often overstated. Some studies link an unforgiving, hostile stance to higher stress-related markers, but the evidence for durable physical-health benefits from forgiveness specifically is thinner and more mixed than popular write-ups suggest. The honest position: holding a grudge has a real, recurring emotional cost that lands mostly on you, and letting go tends to ease that cost — while the bolder bodily claims remain suggestive rather than settled. This is general information, not therapy; for grievances tied to serious harm, a qualified clinician is the right place to work through it.