Inner speech—silent self-directed verbalization—has been hypothesized to play a key role in self-knowledge [5,8,9,10,11], a stable sense of self [12,21], and self-regulation [1,25,27].

Building on these proposals, we examine how inner speech may interact with the symptomatology of Borderline Personality Disorder (BPD), a mental health condition typically emerging in adolescence. So-called ‘self-disturbance'—a poorly defined construct in clinical research—is a core feature of BPD [2,6,14,15,20,26]. Symptoms include feelings of emptiness, impulsivity, affective instability, and stress-related paranoid ideation. Despite extensive research on inner speech, its intersection with BPD remains unexplored. Investigating this could provide insights into the functions of inner speech and the mechanism underlying self-disturbance and related symptoms. 

We hypothesize that individuals with BPD may exhibit inner speech deficits (ISD). This hypothesis warrants further empirical testing and raises important questions:

I. What is the nature of these deficits?

We identify three categories of hypotheses and examine how they can be tested.

1. Vehicle Deficiencies

• Partial or biased inability to generate inner speech may increase the risk of BPD.
• Less frequent use of inner speech than healthy individuals.

2. Content Deficiencies

• More negatively valenced inner speech.
• Less dialogic inner speech.

3. Attitude Deficiencies

• Intrusive inner speech (akin to auditory hallucinations).
• Difficulty trusting one's inner speech.

II. What is the direction of explanation between ISD and BPD symptoms?

Inner speech relies on the development of language, but it also facilitates it [23,16,17,18,19]. However, language development depends on resource allocation strategies optimizing biological goals. Evidence suggests that children prioritizing body growth—e.g., earlier adiposity rebound— exhibit reduced language proficiency at 5–6 years [7]. Similarly, BPD may result from a specific resource allocation strategy responding to early life adversity, facilitating early-age reproduction [4]. We propose five possible ways to explain the relationship between ISD and BPD and assess their plausibility.

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