It is valid criticism. The rule is a rule of thumb, so isn't like a law of physics. The amount of internationalization would be relatively similar over short pe

It is valid criticism. The rule is a rule of thumb, so isn't like a law of physics. The amount of internationalization would be relatively similar over short periods of time, so is an OK measure cf 10-20 years ago, but yes has problems comparing over a century. Other similar issues are the amount of private ownership of US companies (that aren't included in Total Market Cap), and also foreign ownership of US companies which is higher than it used to be (I think).