Can journalists really promise anonymity to their sources?

Investigative journalists rely heavily on their ability to attract and retain the trust of their sources. But can they really promise them anonymity?

Whether a journalist's sources have information about politicians, their policies (or any other information they would prefer goes undiscovered), they will typically only communicate with journalists who promise to keep their identities a secret. The consequences of a journalist failing to uphold such a promise could lead to the end of careers, or worse.

It is often considered that the protection of a source's identity adds to a 'healthy democracy',¹ in that it allows for the free-flow of 'inconvenient truths' which politicians, business people and other powerful members of society would prefer to remain secret.