When Self-Protection Backfires: Adaptive Contact Behavior Expands the Endemic Basin in an Addiction Model with Nonlinear Relapse

We extend the Susceptible--Addicted--Reformed (SAR) model of \cite{sanchez2023}, which exhibits a forward--backward bifurcation driven by nonlinear relapse, by embedding an epi-economic behavioral layer in the spirit of \cite{fenichel2011}.

At-risk individuals choose contact levels by solving a finite-horizon dynamic program that balances the utility of social activity against the expected cost of addiction.

We prove that the basic reproduction number \( R_0 \) and the local stability of the addiction-free equilibrium remain unchanged by the behavioral layer.

However, the endemic structure is fundamentally altered: the behavioral response collapses exactly to a scalar mixing factor \( M \), and the bifurcation curve factorizes as \( R_0(a) = R_{\rm cl}(a)/M(a) \).

This yields an exact comparison principle: the saddle-node fold shifts to lower \( R_0 \) (enlarging the endemic basin) if and only if \( M \ge 1 \) along the branch.

For rational self-protective behavior under conditional proportional mixing, we prove \( M \ge 1 \), so the basin enlarges; the opposite holds under frequency-dependent mixing.

Numerical continuation shows that, at baseline parameters, the fold moves left by \( \Delta R_0 \approx -0.035 \) and the critical initial addiction level drops by 2--6 percentage points.

Gillespie simulations confirm that the enlarged basin increases the stochastic probability of addiction establishment by up to threefold near threshold.

This counterintuitive result that rational self-protection can make addiction easier to establish has direct implications for prevention policy.