In fermion's path integral we have a meassure that you can write, in terms of the Grassman variables $\psi, \bar{\psi}$ as

$$
D\bar{\psi}D\psi, \quad \psi(x) = \sum_n a_n\phi_n(x), \quad \bar{\psi}(x) = \sum_n \bar{a}_n \bar{\phi}_n(x)
$$

Where $a_n, \bar{a}_n$ are Grassman variables and $\phi_n(x)$ a set of orthonormal functions such that

$$
\int d^3x\ \phi^\dagger_n(x)\phi_m(y) = \delta_{nm}
$$

Now if you perform a change of variables in, for instance, axial group $U(1)_A$ with an small parameter $\alpha(x)$, this renders

$$
a'_m = \sum_n(\delta_{mn} + i\int d^3x\ \alpha(x)\phi^\dagger_m(x)\gamma^5\phi_n(x))a_n = \sum_n(1 + C)_{mn}a_n
$$
$$
\bar{a}'_m = \sum_n(1 + C)_{mn}\bar{a}_n
$$
$$
C_{mn} = i\int d^3x\ \alpha(x)\phi^\dagger_m(x)\gamma^5\phi_n(x)), \quad 1\ {\rm is\ the\ identity}
$$

Now, following Peskin (chapter 19.2, Eq. (19.69)), the path integral meassure should change as

$$
D\bar{\psi}'D\psi' = D\bar{\psi}D\psi·(det[1 + C])^{-2}
$$

I don't understand where the -2 power for the jacobian ($det[1 + C]$) came up since if we were talking about a usual integral with usual variables we would end up with +2 power.

What am I missing?