When my son was born, I was determined not to become one of those typical linguist-parents. You know the type: they diligently write down Every Single Utterance produced by their offspring (no matter how seemingly insignificant) and their work suddenly shows a skew towards language acquisition that was previously entirely absent. That was not going to be me. I was going to enjoy fatherhood to the fullest. No work, all play.

Sure, there are things you can't help noticing and find amusing: left branch extractions ("Daddy, how are you tall?"), a continuous struggle to assign nouns to the correct gender (even with exceptionless classes like diminutives), and my goodness, past participle formation is a bitch! I let it all slip, though: this was not a linguistic interview with a native speaker, it was a (usually dinosaur-related) play session with my three-year old son.

But then he started doing something that I just cannot let slide. Here's what he said earlier today:

1. 1. azz-e
  2. if-pl
2. 1. wij
  2. we
3. 1. thuis
  2. home
4. 1. zijn
  2. are
"when we're home"

Note the plural agreement ending on the complementizer. This is a phenomenon conveniently known as comp(lementizer)-agreement in the linguistics literature. It's a phenomenon I've published on and given talks about, i.e. it's something I have a profound professional interest in. And the thing is: Elias shouldn't be doing this. He's born and raised in a non-comp-agreement area, by non-comp-agreeing parents, and surrounded by similarly non-comp-agreeing friends and family. To drive home this point, here's a map of Elias's home vs. the (relevant) comp-agreeing part of the Low Countries:

In other words, he's not just imitating what he hears in his direct surroundings, nor is comp-agreement a surfacy curiosity situated at the very fringe of grammar and not worthy of serious theoretical investigation. There's something deep about this phenomenon, something that indeed warrants the attention it has received in the generative literature. Given that I remain committed to not becoming a linguist-parent, I will not be using Elias's data in any upcoming publications or talks (nor will I be prodding him for more data), but I do want to thank him for suggesting that daddy might be onto something.

And a great source for meta-conversations like the following:
Elias: We've swimmed this morning, haven't we daddy?
Me: Swum. We've swum.
Elias: What is "swum"?
Me: Uhm, it's like swim, but uhm...
Elias: No, we've not swum, we've swimmed!
Me: Okay then.
One possible source of comp-agreement is his best friend's father, who is Dutch and so might be a speaker of a comp-agreement dialect. I haven't heard him use it, but I'll be listening closely next time we meet.
Comp-agreement comes in various shapes and sizes. What I've mapped here is comp-agreement ending in sjwa in the first person plural, i.e. of the type Elias was using.
Nor is he simply copying—via some process of analogy—the verbal affix onto the complementizer, as he's even using comp-agreement in so-called double-agreement contexts, whereby the complementizer and the verb have a different ending:
1. 1. 1. azz-e
    2. if-pl
  2. 1. wij
    2. we
  3. 1. thuis
    2. home
  4. 1. kom-t
    2. come-pl
2. "when we're coming home"