Our main empirical finding is that though two biases, avoiding center-embedding and favoring shorter dependencies, are conceptually similar (both favor simpler grammars), often they capture different aspects of syntax, leading to different grammars.
we take an EM-based simplest approach, and multiply the original model by a constraint factor f (z, x) ∈ [0, 1] to obtain a new model: and then optimize θ based on p (z, x|θ).