IPsec with PSK: Can PSK be used for passive listeners?

I use a PSK to connect to an IPsec VPN.

Let's say an attacker can get access to my PSK.

Then he can pretend to be me, that's clear.

But could she also decipher the intercepted traffic? Or at least has an advantage compared to an attacker who does not know the PSK?
I have a hard time understanding the standard.