4. The computer-implemented method of claim 1 , further comprising: calculating, for each of the candidate topics, a count at which the candidate topic co-occurs with the topics identified from the user profiles of the users in the demographic group; ranking, by the computer, the candidate topics for the target user profile based on the count at which each of the candidate topics co-occurs with topics included in the target user profile; and wherein selecting the candidate topic to add to the target user profile of the target user from the demographic group is based on a rank of the candidate topic in the ranking of candidate topics for the target user profile.
4. The computer-implemented method of claim 1 , further comprising: calculating, for each of the candidate topics, a count at which the candidate topic co-occurs with the topics identified from the user profiles of the users in the demographic group; ranking, by the computer, the candidate topics for the target user profile based on the count at which each of the candidate topics co-occurs with topics included in the target user profile; and wherein selecting the candidate topic to add to the target user profile of the target user from the demographic group is based on a rank of the candidate topic in the ranking of candidate topics for the target user profile. 6. The computer implemented method of claim 4 , wherein the ranking of the candidate topics is based in part on a topic strength between the candidate topic and the user profiles of the users from the demographic group in which the candidate topic co-occurs with the identified topics, the topic strength between the candidate topic and a user profile of a user based on at least one of: a frequency of the user interacting with a digital content item associated with the candidate topic; a duration of at least one interaction of the user with a digital content item associated with the candidate topic; and an amount of time elapsed since a last user interaction of the user with a digital content item associated with the candidate topic.