https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract found that isolated rats took more heroin ea

https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract found that isolated rats took more heroin earlier than rats in "group housing". It's not clear from the abstract how the "group housing" compares to "Rat Park", but it may be the case that even better housing would delay heroin ingestion even longer. Rat Park has been under-studied.