Market or Markets? Investigating Google Search's Market Shares Under Horizontal and Vertical Segmentation

Is Google Search a monopoly with gatekeeping power? Regulators from the US, UK, and Europe have argued that it is based on the assumption that Google Search dominates the market for horizontal (a.k.a. "general") web search. Google disputes this, claiming that competition extends to all vertical (a.k.a. "specialized") search engines, and that under this market definition it does not have monopoly power. In this study we present the first analysis of Google Search's market share under both horizontal and vertical segmentation of online search. We leverage observational trace data collected from a panel of US residents that includes their web browsing history and copies of the Google Search Engine Result Pages they were shown. We observe that Google Search receives 71.8% of participants' queries when compared to other horizontal search engines, and that participants' search sessions begin at Google greater than 50% of the time in 24 out of 30 vertical market segments (which comprise almost all of our participants' searches). Our results inform the consequential and ongoing debates about the market power of Google Search and the conceptualization of online markets in general.