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.

翻译：谷歌搜索是否具有守门人力量的垄断？美国、英国和欧洲的监管机构基于谷歌搜索主导横向（即“通用”）网络搜索市场的假设，认为其构成垄断。谷歌对此提出异议，声称竞争应延伸至所有纵向（即“专业化”）搜索引擎，并在此市场定义下其不具备垄断力量。本研究首次对在线搜索在横向与纵向细分下的谷歌搜索市场份额进行分析。我们利用从美国居民小组收集的观测追踪数据，该数据包含其网络浏览历史及所展示的谷歌搜索结果页面副本。研究发现：与其他横向搜索引擎相比，谷歌搜索接收了参与者71.8%的查询请求；在30个纵向细分市场中有24个（覆盖参与者几乎全部搜索行为）中，超过50%的参与者搜索会话始于谷歌。我们的研究结果为当前关于谷歌搜索市场力量及在线市场概念化的重要讨论提供了实证依据。