Tracking identifier synchronization

A tracking identifier synchronization system using a convergent selection method comprises first party instructions accessed through a content provider and a synchronization system comprising a receiver, a composer and an output. First party instructions direct the client to submit a request comprising a third party cookie to a synchronization system. The composer generates setting instructions for execution at the client. Setting instructions comprise instructions to use a client-executed convergent selection method to select values first party cookies and, optionally, reporting instructions directing the client to transmit the values of client tracking identifiers. Repeated invocation of client-executed convergent selection methods results in the convergence of cookie values for many operational cases. Optionally, the synchronization system may generate third party cookie(s) using a convergent selection method based on the reporting data and compose a second set of third party cookie setting instructions for execution at the client.