Andrew Carnie. Syntax, A Generative Introduction (3 ed, 2012). p 25.
Consider the fourth sentence in the paradigm in (28). This sentence is the same as (28c) but with a that:
d) *Who do you think that _____ will question Seamus first?
It appears as if that is only optional when the question word (who in this case) starts in object position (as in 28a and b). It is obligatorily absent when the question word starts in subject position (as in 28c and d) (don’t worry about the details of this generalization). What is important to note is that no one has ever taught you that (28d) is ungrammatical. Nor could you have come to that conclusion on the basis of the data you’ve heard. The logical hypothesis on the basis of the data in (28a–c) predicts sentence (28d) to be grammatical. There is nothing in the input a child hears that would lead them to the conclusion that (28d) is ungrammatical, yet every English-speaking child knows it is. One solution to this conundrum is that we are born with the knowledge that sentences like (28d) are ungrammatical.8 This kind of argument is often called the underdetermination of the data argument for UG.
8 The phenomenon in (28) is sometimes called the that-trace effect. There is no disputing the fact that this phenomenon is not learnable. However, it is also a fact that it is not a universal property of all languages. For example, French and Irish don’t seem to have the that-trace effect. Here is a challenge for those of you who like to do logic puzzles: [MY QUESTION] If the that-trace effect is not learnable and thus must be biologically built in, how is it possible for a speaker of French or Irish to violate it? Think carefully about what kind of input a child might have to have in order to learn an “exception” to a built-in principle. This is a hard problem, but there is a solution. It may become clearer below when we discuss parameters.
I can't remember where in this book, but author analogizes Universal Grammar to checklist of features. Then isn't answer to question that 'that-trace effect' is deselected in French and Irish's checklist? Where's the "logic puzzle"?