According to the theory of the sandbox effect, links which may normally be weighted by Google's ranking algorithm, not least improving the position of a webpage in Google's index, may be subjected to filtering to prevent their having a full impact. Some observations have suggested that two important factors for causing this filter to come into play are the active age of a domain, and the competitiveness of the keywords used in links.
Active age of a domain should not be confused with the date of registration on a domain's WHOIS record, but instead refers to the time when Google first indexed pages on the domain. Keyword competitiveness refers to the search frequency of a word on Google search, with observation suggesting that the higher the search frequency of a word, the increasing likelihood that the sandbox filter effect will come into play. While the presence of the Google Sandbox has been long debated, Google has made no direct disclosure on the matter. However, as the sandbox effect almost certainly refers to a set of filters in play for anti-spam purposes, it is unlikely Google would ever provide details on the matter. In one instance though, Google's John Mueller, has mentioned that "it can take a bit of time for search engines to catch up with your content, and to learn to treat it appropriately. It's one thing to have a fantastic website, but search engines generally need a bit more to be able to confirm that, and to rank your site - your content - appropriately". This could be understood as the cause for the sandbox effect.
Google has long been aware that its historical use of links as a "vote" for ranking web documents can be subject to manipulation, and stated such in its original IPO documentation. Over the years Google has filed a number of patents that seek to qualify or minimise the impact of such manipulation, which Google terms as "link spam".