Languages change over time. Most varieties of english no longer distinguish between 'whale' and 'wail' in speech, for example. In french the word for step, 'pas', when used roughly like "whit" in "I don't care a whit", came to lose it's sense of step-ness and become simply a negation marker. While we occasionally see cases where a language becomes more complex, we usually see simplifications. Distinctions between sounds ('wh' vs 'w') disappear. Tricky grammatical rules (it was once common to hear "if I were in charge ..." but now the subjunctive is on the decline and more common is "if I was in charge") give way to simpler ones. My friends learning latin would often claim that latin is harder than it's descendent, spanish, perhaps using case [1] to make their point.

With all these easy examples of language simplification and very few of the reverse, as linguistics students we would ask our teachers: "why do languages get simpler over time?". The standard response would be that "no language is simpler or more complex than another."

This is one reason I was excited to read The Power of Babel. McWhorter argues that the language of a group becomes more complex over time if the group is left alone but that imperfect adult learning is a force in the opposite direction. So many languages of isolated groups are incredibly complex while creoles are much simpler. 'Big' languages like english are somewhere in between: as they have displaced local languages (in the case of english: cornish, welsh, scots, irish, and others) lots of adults have made simplifications in learning the language which they pass on. Similarly, whatever forces that add complexity seem to be either stopped or dramatically slowed when the population is large enough.

So now I understand why we should expect spanish to be easier to learn than latin: spanish is the result of lots of people who originally spoke 'indigenous' languages learning the language of a conquering empire. Many people learned as adults, and not very well, leaving out the harder stuff. Because the things about language that we find difficult are somewhat consistent across languages, many of those same aspects that the people in spain left out are the ones that really drive us crazy when learning latin.

What is the force acting in the other direction? I understand how changes can be faster in smaller groups, as many forces are the same as those that make evolution happen faster on islands. But why do changes that make second language learning more difficult seem to be more common than changes that would make it easier? Is it just a matter of entropy? Perhaps without large scale second language learning keeping the language from getting too hard the entropy factor of "there are more ways to make it hard than there are to make it easy" means that it gets harder? Or is there an advantage to making your language hard for outsiders to pick up in and of itself?

[1] Case is the distinction between 'I' and 'me'. We say "I went to the store' but 'He gave it to me". In english we only have case for pronouns like 'I','me', 'he', and 'she' but in latin pretty much all nouns varied with case. It's as if I had to distinguish between two forms of 'book' with "I don't like that bookobj" versus "The booksubj is dull".