Well, it would be stupid to trust the broker. What technically prevents them to run away with all stocks?
I currently manage it by simply having accounts with several to avoid putting all eggs in one basket. But this is hardly a solution.
That is where your personal judgement comes in of course, but it would be hard for them to "run away with the stocks". These are not physical certificates, but just registrations in a central organisation.
Brokers (in traditional finance) are regulated, audited, and also are often subject to insurance policies in case they went bankrupt, or similar. If you are dealing with a large and reputable institution, you are unlikely to gain any benefit by assigning the shares to yourself I would suspect. Someone please correct me if I am missing something though.
What prevents them is custody. Funds, brokers, etc. are all required to use custody providers to keep track of assets and handle the actual transfer of assets.
> What technically prevents them to run away with all stocks?
It's called the law.
> Well, it would be stupid to trust the broker. [..] I currently manage it by simply having accounts with several[...]
If you believe it's "stupid" to trust brokers, spreading your exposure across several of them may protect against a complete loss. But it also increases the risk of some loss and, on balance, has exactly the same expected value. The law diminishing marginal utility may put your strategy slightly ahead, but the difference is similarly marginal.
