How big is the effect of multilingualism on IQ testing of preschoolers?
As background, we have what I think is probably a moderately gifted kid. He is growing up trilingually, and has been behind in his language skills compared to monolingual kids in all 3 languages. His weakest language is English.Currently, he probably wouldn't even understand testing instructions in English. However, he will have been in an English speaking preschool for 4-5 months when taking the IQ test (presumably WPPSI?) next year at 4 yrs old. Am I correct in fearing that his verbal skills might make enough of a difference to have a moderately gifted kid be classed as not gifted, or only mildly gifted? Thanks!

I think you are correct. Any factor that interferes with compliance (language comprehension, attention problems, hearing issues, disruptive behaviour, etc etc) is going to interfere with the accuracy of the results.

You could have him tested anyway and take the results with a grain of salt, or wait until his English is better (this is assuming you can't have him tested in his strongest language ?)

From what I understand IQ testing at age 4 is pretty unreliable to begin with... not even fluently speaking the language the test is given in would make the whole thing pointless, I'd think.

There are tests that are meant to be better for children who do not speak English as their primary language.. the NNAT, for one. It's not nearly as thorough as most other tests.. it's pretty much just a non-verbal test.

The law for special education requires that tests be given in the child's first language. I don't know what they do if the child is being brought up in 3 languages simultaneously.