BPD and Intellectual Disabilities: A Dual Diagnosis That Is Frequently Missed

She had been receiving services for her intellectual disability since she was a child. Over the years, her team had learned her routines, understood her communication style, and built genuine relationships with her. But in her early twenties, something shifted. The emotional swings became more intense. Relationships felt impossible to hold onto. Small changes in her day could trigger distress that lasted for hours. She would reach desperately for connection and then push people away in the same breath.

Her team adjusted her behavior support plan. They tried new communication strategies. They added more structure to her day. The distress continued.

What no one had considered — not once in years of documentation and team meetings — was that she might be living with borderline personality disorder alongside her intellectual disability.

This is not an unusual story. For individuals with IDD, BPD is one of the most frequently missed co-occurring mental health conditions. And the consequences of missing it are significant.

What Borderline Personality Disorder Actually Is

Borderline personality disorder is a mental health condition characterized by intense and rapidly shifting emotions, a deep fear of abandonment, unstable relationships, an unclear or shifting sense of self, and difficulty regulating feelings and impulses. It is not a character flaw, a choice, or evidence of a difficult personality.

BPD develops most often in response to environments where emotional experiences were consistently invalidated, unpredictable, or unsafe — particularly during childhood and adolescence. The brain learns to respond to the world with heightened sensitivity and urgency because, at some point, that sensitivity was necessary for survival.

Common experiences for people living with BPD include:

• Intense emotional reactions that feel overwhelming and are difficult to bring back down
• A persistent fear that important people will leave or abandon them
• Patterns of idealizing and then feeling deeply disappointed by people they care about
• A fragile or shifting sense of who they are and what they want
• Impulsive behavior during moments of emotional intensity
• Chronic feelings of emptiness or emotional numbness between episodes of distress

These experiences are painful, disorienting, and often isolating. They are also treatable — with the right approach, the right understanding, and the right support.

Why This Diagnosis Is So Often Missed in People With IDD

People with intellectual and developmental disabilities experience BPD at significant rates, yet they are rarely assessed for it. There are several reasons this gap persists, and understanding them is the first step toward closing it.

The most pervasive barrier is diagnostic overshadowing. This is the clinical tendency to attribute all of a person's emotional and behavioral difficulties to their intellectual disability, without considering whether a separate mental health condition might also be present. When a person with IDD experiences intense emotional dysregulation, the assumption is often that this is simply how their IDD presents — rather than a signal that something additional is happening.

A second barrier is that standard BPD assessment tools were not designed for people with cognitive or communication differences. The questionnaires and interview formats used to assess BPD rely heavily on a person's ability to reflect on and articulate abstract emotional experiences. For someone with IDD, this process may be genuinely inaccessible, even when the underlying experience is very real.

A third barrier is stigma. BPD already carries more stigma within clinical settings than almost any other mental health diagnosis. Providers who hold negative assumptions about BPD may be reluctant to apply the diagnosis at all — and even more so when the person already has an IDD diagnosis that can absorb the explanation.

The result is that individuals with IDD and undiagnosed BPD often spend years in behavioral support systems that are not designed to address what they are actually experiencing. They are described as difficult, treatment-resistant, or high-need. They cycle through placements and programs. And they continue to suffer in ways that a correct diagnosis and appropriate treatment could meaningfully address.

What the Overlap Actually Looks Like

When BPD and IDD co-occur, the presentation can look different from BPD in the general population — and knowing what to look for matters.

Emotional dysregulation may be more intense and more visible. A person with IDD and BPD may have fewer internal or verbal tools for managing overwhelming feelings, which means distress often moves directly into behavior. Staff and caregivers may see rapid escalations that seem disproportionate to the trigger, along with an equally rapid return to baseline once the emotional storm passes.

Fear of abandonment may be expressed through behavior rather than words. This might look like extreme distress when a favorite staff member is absent, clinging behavior during transitions, or repeated testing of whether a caregiver will stay. These responses are often interpreted as behavioral problems or attention-seeking rather than as expressions of profound relational fear.

Unstable relationships may appear as cycles of intense connection followed by sudden rejection. A person might speak of a caregiver as the most important person in their life one week, and refuse to engage with them the next. This pattern is frequently experienced by support teams as confusing and demoralizing — and rarely recognized as a hallmark symptom of BPD.

Identity disturbance may be less visible but no less real. A shifting or fragile sense of self can appear as difficulty making decisions, intense responsiveness to how others seem to feel about them, or a deep uncertainty about preferences, values, and belonging.

What Accurate Assessment and Affirming Treatment Require

Getting to an accurate diagnosis for a person with IDD and possible BPD requires clinicians who are willing to look beyond the existing IDD diagnosis and ask deeper questions about emotional experience, relational patterns, and developmental history.

This means using adapted, accessible assessment approaches. It means spending time building enough trust and safety that a person can begin to share their inner experience, even partially and nonverbally. It means gathering information from caregivers, support staff, and family members who know the person's patterns over time. And it means approaching the assessment with curiosity rather than assumption.

Once an accurate diagnosis is in place, treatment for BPD in people with IDD draws on many of the same evidence-based approaches used in the general population — particularly Dialectical Behavior Therapy, known as DBT. DBT was specifically designed for people who experience intense emotional dysregulation, and its core skills — distress tolerance, emotional regulation, interpersonal effectiveness, and mindfulness — can be adapted for different cognitive and communication levels.

Adapted DBT for people with IDD uses simpler language, visual supports, concrete examples, and shorter skill-building sessions. When delivered by a clinician with dual expertise in both BPD and IDD, it can produce meaningful and lasting change.

Equally important is the environment around the person. Caregivers and support staff who understand BPD — who can respond to emotional dysregulation with validation rather than frustration, and who know how to maintain consistent, boundaried relationships — are themselves a form of treatment. The relational environment is not separate from the clinical work. It is part of it.

What Families and Support Teams Can Do

If you support or love someone with IDD who you believe may be experiencing more than their IDD diagnosis accounts for, you are not imagining it and you are not alone.

Here are meaningful steps you can take:

• Advocate for a comprehensive mental health evaluation that goes beyond behavioral assessment and considers co-occurring conditions including BPD
• Share patterns you have observed over time — the emotional cycles, the relational dynamics, the responses to perceived abandonment — with the clinical team
• Ask specifically whether diagnostic overshadowing might be influencing how the person's distress is being interpreted
• Seek providers with experience in both IDD and personality disorder treatment, as the combination of expertise makes a significant difference in outcomes
• Learn about validation as a daily response strategy — meeting the person's emotional experience with acknowledgment rather than correction is one of the most powerful tools available
• Take care of yourself — supporting someone with BPD and IDD is genuinely demanding work, and your own emotional regulation directly affects the person you are supporting

A correct diagnosis is not a ceiling. It is a door. When someone with IDD finally receives an accurate BPD diagnosis and access to appropriate treatment, the change in how they understand themselves — and how the people around them understand them — can be profound.

If you or someone you support is in crisis, please reach out:

If someone is in immediate danger, call 911.
‍988 Suicide and Crisis Lifeline: Call or text 988
Crisis Text Line: Text HOME to 741741
SAMHSA National Helpline: 1-800-662-4357