Consider the analogous hypotheses about attractive real estate brokers, which are in higher demand. You might claim that broker attractiveness is just an indicator of broker quality. One hypothesis says attractive brokers are in more demand mostly because they sell houses faster, while another hypothesis says the are in more demand mostly because people like to hang out with attractive people. We could compare these theories empirically by seeing if in fact attractive brokers sell houses faster or for more money. If they don't, or if their gains aren't enough to cover their added costs, the second hypothesis would be favored.