(The PDA conference in Perth on November 11, 2025)
Kate Heina is an occupational therapist who has worked with neurodivergent people for 16 years. She’s also the mother of a child with a PDA profile. In her talk, she explains the difference between capability and capacity, and how we should match what we expect from a child with what they can actually handle at the moment.
Starting point
The talk is grounded in Ross Greene’s idea: kids do well if they can. When we speak of “difficulty,” we do not mean willful defiance; we mean that the demands placed on the child exceed what they can handle at that moment. Behavior shows whether a child is coping with pressure, both the kind they place on themselves and the kind that comes from outside.
The nervous system and safety
Kate grounds this line of thinking in neurobiology, drawing on Stephen Porges’ polyvagal theory and the concept of neuroception — the automatic, non-conscious monitoring of the environment for safety.
The autonomic nervous system functions independently of conscious control. In this case, it is not a decision but a physiological state.
When the system detects safety, executive functions such as learning, planning, communication, and relationship-building become more accessible. When threat is detected, the stress response activates and access to the “thinking part of the brain” becomes limited.
For people with a PDA profile, the threat-detection system might be especially sensitive. Stressors like demands, deadlines, loss of control, and violations of autonomy might all show up as threat cues.
Cumulative stress and the “bucket”
Kate refers to the idea of cumulative stress, described in Stuart Shanker’s work. Stress keeps building. The metaphor of the bucket represents the total energy available, or the capacity to deal with everyday demands.
Demands and pressure act like a running tap. Every additional burden adds more weight to the bucket. Stressors accumulate and gradually narrow the window of tolerance — a term popularized by Dan Siegel to describe the range of optimal arousal.
As long as the load remains within a manageable range, the child can access their capabilities. When the bucket overflows, the system shifts into a fight-or-flight state. This can also appear as freeze or shutdown.
Capability versus capacity
The word capability means that a given skill is present.
Capacity refers to the regulatory resources available to use that skill at a given moment. One day a person may be able to do a great deal, and the next day much less. That may be because they are tired, ill, stressed, or carrying the effects of earlier strain.
Skills do not simply disappear. They may become temporarily inaccessible. In that sense, this is about can’t, not won’t. Capacity is determined by the current state of the nervous system, not by willpower.
Aligning expectations
If capacity is variable, expectations must also remain flexible. Kate uses the phrase aligning expectations, meaning adjusting expectations to what is currently possible.
This shifts the focus away from applying more pressure and toward adapting demands to the child’s current regulatory state. It is about understanding the cost of different tasks and recognizing when it is possible to gently stretch capacity (reach up) and when it is better to ease off (drop).
Three levels of capacity
Kate suggests three approximate levels of capacity:
1. Very low capacity / Recovery
There is very little capacity. Functioning is limited to basic self-care and the safest possible environment. Expectations need to remain low. Recovery takes time and cannot be rushed.
2. Stability
Moderate capacity. It becomes possible to gradually expand the range of safe people, places, and activities.
3. Stretch days
There are periods when increased capacity becomes possible and more demanding activities can be undertaken. These periods require limiting other burdens in advance and allowing for substantial recovery afterward.
These levels can go up or down.
Monitoring the here and now
In addition to capacity, it is important to keep checking whether the child is still within their window of tolerance. The key question becomes: Are we close to the tipping point of overwhelm?
Here are some early signs that your capacity might be dropping:
- procrastination,
- retreat into fantasy,
- more sensory sensitivity
- greater need for control
- changes in communication
- more movement and physical tension.
In each moment, you are deciding whether to support reach up or move toward drop. It is not a rigid procedure or algorithm, but a process that requires attention and attunement.
Ending
Finding the right balance between capability and capacity is not straightforward. Mistakes happen. The core idea is that trust matters. Children will do what they can when they are ready, not when they are under pressure.
PS. Who does Kate Heina refer to?
Ross W. Greene
A clinical psychologist and the creator of the Collaborative & Proactive Solutions (CPS) model. He is the author of the phrase “Kids do well if they can,” which posits that challenging behaviors stem from missing skills or overload, not from bad intent.
Dr. Stephen Porges
A neurophysiologist and the originator of the polyvagal theory. He describes how the autonomic nervous system responds to cues of safety and threat, and how these responses shape access to cognitive and relational functions.
Dan Siegel
A psychiatrist and a researcher in the field of interpersonal neurobiology. He popularized the concept of the “window of tolerance,” which describes the range of arousal in which optimal cognitive and emotional functioning is possible.
Stuart Shanker
A developmental psychologist whose research is focused on self-regulation. He characterizes stress as cumulative, emphasizing that behavior serves as an indicator of the load carried by the nervous system.