Reportedly the syndrome "occurs in 8% of kidnap victims", by FBI stats. Not sure how they measure that, but seems plausible. Of course when forced to act against your will, you're defiant. Fight or flight. That's what happens most of the time. Except it's not so simple.
There's a fuller description of the strategies known as "Fight, Flight, Freeze, Fawn" (I guess it was super important to to keep the F-s, hehe).
We see the Freeze response in nature, deer stuck in headlights, animals pretending to be dead when attacked, and so on. We can see this response in children with violent parents. They can't run and hide, nor fight. They freeze.
Fawn is basically the "Stockholm syndrome":
- Over-agreement
- Trying to be overly helpful
- Primary concern with making the abuser happy
This 8% figure with kidnapping seems to be low because the "Fawn" adaptation takes time to develop. A kidnapping is sudden and unexpected. No time to adapt. But there are abusive situations when there is plenty of time to adapt.
We can see this in cults, in abusive families, autocratic companies, it's pervasive in fascist regimes, i.e. Jews policing, hating and attacking Jews, etc.
Fawn is the default adaptation when an abuser is abusive over a long period of time, gradually going from non-abusive to abusive, like in an abusive marriage, from honeymoon to everyday scandals. The victims seek to align to an increasingly lopsided point of balance by changing themselves. And over time, it can become absurd.
To wit, the second most prominent supposed case of Stockholm Syndrome was the kidnapping of Patty Hearst, who was supposedly held captive by the SLA terrorists and abused for a protracted period of time before she started helping them rob banks.
Kapos, collaborators or hostages joining the cause of their captors are not a "syndrome" of people "falling in love" or "bonding" with their captors. Patty Hearst was just convinced of the cause of the SLA - and she tried to get out of jail by using that card. She was just neither bright not honest - a spoiled brat jumping on an adventure that turned bad.