Ancestry Approach

The ancestry approach is a supplementary search strategy used in systematic reviews to identify additional relevant studies by examining the reference lists of included articles and key review papers. Also known as backward citation searching or reference list checking, it traces the intellectual lineage of cited sources to capture studies that may have been missed by electronic database searches.

No single database search, however comprehensive, captures every relevant study on a topic. Studies may be missed because of inconsistent indexing terms, publication in non indexed journals, language barriers, or the use of unconventional terminology. The ancestry approach provides a practical and efficient way to fill these gaps, and it is part of standard systematic review practice: Cochrane's methodological standards for intervention reviews (MECIR) and the JBI manual both expect reference lists to be checked, and the PRISMA 2020 reporting guideline asks reviews to report citation searching as an information source. Empirical studies have shown that reference list checking identifies additional eligible studies in a substantial proportion of systematic reviews, sometimes uncovering evidence that materially affects the review's conclusions.

The ancestry approach is typically conducted after the primary database search is complete and the initial set of eligible studies has been identified. The reviewer examines the reference list of each included study, scanning titles and, where necessary, abstracts to identify potentially relevant citations. Studies that appear to meet the review's eligibility criteria are retrieved and assessed against the full inclusion and exclusion criteria.

This process can be applied iteratively. When a newly identified study is included in the review, its reference list is also examined, and the cycle continues until no further relevant studies are found. In practice, reviewers often stop after one or two rounds, once a round finds nothing new.

The ancestry approach is most effective when combined with forward citation searching (sometimes called the descendancy approach), which identifies newer publications that cite the included studies. Forward citation searching is conducted using citation databases such as Web of Science, Scopus, or Google Scholar. Together, backward and forward citation searching create a bidirectional net that captures studies published both before and after the included studies.

The method does have limitations. It can only identify studies that have been cited by the included studies or reviews, so it will miss studies that exist in isolation from the citation network, such as unpublished reports or studies in non indexed local journals. It can also introduce a form of citation bias if the included studies preferentially cite work that supports their own findings. For these reasons, the ancestry approach should supplement, not replace, comprehensive database searching.

The ancestry approach should be a standard part of a systematic review search, used to supplement database searching rather than to replace it. It is particularly valuable in fields where the literature is scattered across many journals and databases, where terminology is inconsistent, and where older foundational studies may not be well indexed in electronic databases. It is less critical for reviews on narrowly defined, well indexed topics where database searches have high sensitivity.

A common mistake is relying on the ancestry approach as the primary search strategy rather than a supplement, which can introduce selection bias because the studies identified depend entirely on what the included studies chose to cite. Another error is failing to document the ancestry search process transparently. PRISMA 2020 asks authors to report all information sources, and reviewers should specify which studies and reviews were checked, how many additional studies were identified, and how many were ultimately included.

Some reviewers apply the ancestry approach inconsistently, checking reference lists for some included studies but not others, which can introduce bias. The approach should be applied systematically to all included studies and to any relevant review articles identified during the search. Finally, reviewers sometimes confuse the ancestry approach with a general literature browse; the method requires structured screening against predefined eligibility criteria, not casual reading of reference lists.

A systematic review on the effectiveness of school based mental health interventions identifies 34 studies through searches of PubMed, PsycINFO, ERIC, and the Cochrane Library. The review team then examines the reference lists of all 34 included studies and three previously published systematic reviews on the topic. This process yields 11 additional potentially relevant studies, of which seven meet the eligibility criteria after full text review. Two of these seven studies were published in education journals not indexed in the medical databases searched, and one was a government report available only through an institutional repository.