The Mozart effect: what is left of a beautiful hypothesis
The IQ boost was real, lasted fifteen minutes, and had nothing to do with Mozart.
In 1993 a short paper reported that students who listened to ten minutes of a Mozart sonata did better on a spatial reasoning task. The effect lasted about a quarter of an hour.
The press turned this into the claim that Mozart raises intelligence, especially in infants. In some US states maternity hospitals were supplied with the recordings at public expense.
Checks showed something different. The improvement came from any music the listener enjoyed and that lifted their arousal — in experiments a Stephen King story worked just as well. The cause is emotional arousal and mood, not a particular composer.
That said, the link between music and cognition is real, just different: years of playing an instrument are associated with better working memory and auditory attention. Listening is not enough — you have to play.