This paper uses a database covering the universe of French firms for the period 1990-2007 to provide a forensic account of the role of individual firms in generating aggregate fluctuations. We set up a simple multi-sector model of heterogeneous firms selling to multiple markets to motivate a theoretically-founded set of estimating equations that decompose firms' annual sales growth rate into different components. We find that the firm-specific component contributes substantially to aggregate sales volatility, mattering about as much as the components capturing shocks that are common across firms within a sector or country. We then decompose the firm-specific component to provide evidence on two mechanisms that generate aggregate fluctuations from microeconomic shocks highlighted in the recent literature: (i) when the firm size distribution is fat-tailed, idiosyncratic shocks to large firms contribute to aggregate fluctuations (the "granularity" hypothesis of Gabaix, 2011), and (ii) sizable aggregate volatility can arise from idiosyncratic shocks due to input-output linkages across the economy (Acemoglu et al., 2012). We find that firm linkages are approximately twice as important as granularity in driving aggregate fluctuations.
