Firms that have a high BE/ME (a low stock price relative to book value) tend to have low earnings on assets. Conversely, low BE/ME (a high stock price relative to book value) is associated with persistently high earnings.
Although size and book to market equity seem like ad hoc variables for explaining average stock returns, we have reason to expect that they proxy for common risk factors in returns.
If assets are priced rationally, variables that are related to average returns, such as size and book-to-market equity, must proxy for sensitivity to common (shared and thus undiversifiable) risk factors in returns. The time-series regressions give direct evidence on this issue. In particular, the slopes and R2 values show whether mimicking portfolios for risk factors related to size and [book-to-market] capture shared variation in stock and bond returns not explained by other factors.