Rubber has a Poisson's ratio close to 0.5.

Poisson's ratio: $\sigma = \dfrac{\Delta d/d}{\Delta L/L}$, between $0$ and $0.5$. A stretched rod's volume changes by $\dfrac{\Delta V}{V} = \varepsilon(1 - 2\sigma)$. The moduli are linked: $Y = 2G(1 + \sigma)$ and $Y = 3B(1 - 2\sigma)$.