Why do some people feel better just by believing they've taken an antidepressant, while others feel nothing - or even worse? A new study from the University of Marburg and Harvard Medical School suggests the answer lies in our personality traits and how our brains process dopamine, the chemical messenger tied to motivation and reward.
The study design
In a double-blind experiment with 293 healthy volunteers, researchers tested both a real drug and a placebo. Participants were randomly given either a dopamine D2 receptor blocker called sulpiride or an inactive pill. At the same time, they were told either that the pill was an antidepressant expected to lift mood, or that it was an inert substance. Over the course of six hours, participants reported their levels of positive affect - feelings of energy, interest, and pleasure.
The scientists also measured two key personality traits: anhedonia (the tendency to feel little pleasure in activities) and extraversion (the tendency to be outgoing and experience frequent positive moods). These traits are known to be tied to dopamine signaling.
What they found
The results were striking:
- High-anhedonia participants (those who normally struggle to feel pleasure) reported increases in positive affect after taking sulpiride. In other words, the drug gave them a lift.
- Low-anhedonia participants (those who already felt more positive) actually felt worse after sulpiride. The same drug pulled their mood down.
- When it came to expectations, the pattern flipped along the extraversion dimension: introverts (low extraversion) felt better when they believed they had taken an antidepressant. Extraverts, however, reported lower positive affect under the same expectation.
In short: both the real drug and the placebo effect boosted mood for people with low baseline positivity, but flattened it for people with higher baseline positivity.
Why does this happen?
The brain's dopamine system seems to be finely tuned. For people starting out "low" (anhedonic, introverted), a push - whether chemical or psychological - creates a real lift. For people starting out "high" (extraverted, naturally positive), that same push destabilizes the balance, leaving them worse off.
It's not that sulpiride is universally helpful or harmful. Instead, it acts like a leveler: raising the floor for those stuck low, while lowering the ceiling for those already high. Placebo expectations work in much the same way.
This echoes a long-standing observation in psychiatry: many antidepressants don't outperform placebo by much in clinical trials. What seems to matter more is the fit between the person's baseline dopamine traits and the kind of intervention they receive.
A challenge to the idea of a "universal" antidepressant
The study's authors note that their findings complicate the idea of one-size-fits-all depression treatments. If medication can raise mood in one person while lowering it in another, then the key is not the pill alone but the interaction between the pill, the expectation, and the person's dopamine-related traits.
This may help explain why some patients benefit dramatically from antidepressants, while others feel no change - or only side effects. It also highlights why placebo responses can be so powerful: for those with low baseline dopamine tone, the thought of healing itself becomes rewarding.
A clarification: introversion is not anhedonia
One caution is important. In this study, low extraversion often lined up with higher anhedonia, and high extraversion with lower anhedonia. But that's a statistical link, not a rule for individuals.
Being introverted does not automatically mean being anhedonic. Many introverts find deep joy in solitary or intellectual pursuits - reading, creating, imagining, solving problems. They may light up inside from activities that don't show up on standard personality questionnaires. Conversely, anhedonia means losing pleasure across the board, including in those inner activities.
So while researchers often pair "high anhedonia = low extraversion," real life is more nuanced. An introvert can still be low-anhedonia, thriving on inner pleasures, just as an extravert can experience high-anhedonia, chasing stimulation but not enjoying it.
