Sexual Brakes and Accelerators: Nagoski's SIS/SES Model Applied to Desire

What are sexual brakes and accelerators?

The model was originally developed by John Bancroft and Erick Janssen at the Kinsey Institute under the name the dual control model of sexual response. Emily Nagoski popularized it in Come As You Are, where she applied it to explain the full range of human sexual variation — not as deviations from a healthy baseline, but as different configurations of the same two-system architecture.

The central insight is that problems with desire are almost never caused solely by a weak accelerator. They are usually caused, at least in part, by a strongly activated or highly sensitive brake. Most conventional advice about desire — more stimulation, better technique, scheduled intimacy — aims at the accelerator and ignores the brake entirely. That is why so much of that advice fails.

The sexual excitation system (SES): what activates desire

The SES is the accelerator. It scans the environment for sexually relevant stimuli and responds to them by activating arousal circuits. Dopamine is among the key neurotransmitters involved, generating wanting and approach motivation. Testosterone amplifies excitatory sensitivity — the higher the baseline testosterone, the lower the threshold for stimuli to trigger SES activation.

What the SES responds to varies substantially across people. For many, touch is the primary excitatory signal — the right kind of physical contact activates the system relatively directly. For others, emotional intimacy is the primary activator: feeling genuinely known and desired by a specific person generates more excitatory signal than any physical technique. For others, novelty matters most: new experiences, variation, or the erotically charged uncertainty of an early encounter amplify SES activity in ways that routine does not.

Visual and scent cues activate the SES for many people. Fantasy activates it for most, because the SES responds to imagined erotic stimuli as well as real ones. The erotic imagination is genuinely excitatory — not merely a substitute for real contact but a direct input into the system. This is why fantasy life matters even in the context of a relationship, not as escape from the partner, but as active engagement of the excitatory system.

Individual variation in SES sensitivity is real and substantial. Some people have highly responsive excitatory systems that activate easily across a range of stimuli. Others have systems that require more specific, more sustained, or more contextually appropriate input before activation occurs. Neither configuration is healthier. They simply require different approaches to function well.

The sexual inhibition system (SIS): what suppresses desire

The SIS is the brake. It scans for potential threats — physical danger, social risk, performance threat, relational instability, shame — and responds by suppressing sexual arousal. This makes evolutionary sense. An organism mid-coitus is highly vulnerable. A nervous system that could not shut down sexual activity in response to genuine threat would be at serious disadvantage.

The problem in modern contexts is that the SIS does not reliably distinguish between genuine physical danger and the stress of a deadline, an uncomfortable conversation, or a child who might walk in. It responds to perceived threat, and many features of contemporary life read as threatening to a sensitive inhibitory system. The result is a brake that is frequently engaged not because sex is actually dangerous but because the nervous system has not received a clean signal that it is safe.

The SIS has two primary sub-components, identified by Bancroft and Janssen as SIS1 and SIS2. SIS1 inhibits sexual response in response to threat of performance failure — the fear of not performing adequately, not being aroused enough, not satisfying a partner, or being humiliated in some sexual context. SIS2 inhibits sexual response in response to threat of consequences — pregnancy risk, disease risk, being caught, relationship damage, or social embarrassment. Different people are more sensitive to one versus the other, which produces different inhibitory profiles.

High SIS1 sensitivity tends to produce significant performance anxiety, self-monitoring during sex, and difficulty maintaining arousal under evaluation. High SIS2 sensitivity tends to produce desire suppression in contexts where sex feels risky, inappropriate, or potentially consequential. Both produce brake activation, but through different trigger mechanisms.

How the two systems interact in responsive desire

Responsive desire sits primarily on the brake side of the model. It is not that the accelerator is weak — it is that the brake needs to release before the accelerator can generate desire from a neutral state. The person with responsive desire and a sensitive SIS may have robust excitatory capacity. But that capacity cannot express itself until the inhibitory system has received enough reassurance, comfort, and safety signal to stop suppressing.

This is why the sequence matters so much. Spontaneous desire moves from excitation to approach. Responsive desire moves from reduced inhibition to excitation to approach. The middle step — reducing inhibition — is not a failure of desire. It is a requirement of the mechanism.

When couples understand this, they stop treating the absence of spontaneous readiness as the problem and start treating elevated brake activation as the information. What is engaging the brake right now? Is it the relational atmosphere? The week's stress? Physical discomfort? Performance anxiety? Once the brake is identified, it can be addressed. Once the brake releases, the accelerator — which was working fine all along — does its work.

Your personal brake-to-accelerator ratio

The practical goal of understanding the dual control model is not to categorize yourself into a type but to develop genuine self-knowledge about your specific configuration. How sensitive is your accelerator, and to what stimuli specifically? How sensitive is your brake, and to what threats specifically? Which of the two systems is more variable, more responsive to context changes, more amenable to deliberate modification?

Most people with desire difficulties discover that their brake is more worth attending to than their accelerator. The excitatory system tends to be more constitutionally stable. The inhibitory system tends to be more environmentally responsive — more shaped by current relationship quality, current stress load, current hormonal state, and current psychological weight. Working on inhibitory load therefore tends to produce faster and more durable results than amplifying stimulation.

That said, understanding your personal accelerator profile is not trivial. Many people have never actually investigated what specifically activates their sexual excitation system rather than assuming they already know, or assuming it should be the same as their partner's, or defaulting to cultural scripts about what should be arousing. Personal accelerator literacy is a genuine form of self-knowledge, and it tends to be worth the exploration.

For how attachment style shapes the specific threats your SIS is most sensitive to, see Responsive Desire and Attachment Style. For the foundational definition and context of responsive desire within this model, see What Is Responsive Desire.