There are some key differences between schizoid personality disorder (SPD) and simply having a low affect or muted emotional expressiveness:

• Cause – SPD is a clinical condition with a psychological basis, while low affect may be an inherent personality trait not tied to a disorder.
• Severity – SPD typically involves extremely limited emotional expression and disjointed thought patterns well outside the norm, whereas low affect refers to below average but not pathological emotional range.
• Distress – People with SPD often have difficulty forming relationships and can experience distress about their degree of detachment. Those with mere low affect typically don’t have severe impairment.
• Motivation – SPD involves lack of desire for social and emotional connections. With low affect, the desire may exist but expression remains muted.
• Flexibility – SPD reflects rigid, lifelong patterns, while low affect can be situational or fluctuate based on mood and context.
• Internal experience – Those with SPD may have limited inner emotional experience as well. People with low affect can still feel emotions internally but not outwardly show them.

So while the two can appear similar on the surface, SPD represents more pervasive and debilitating challenges with emotional restriction along with an inward sense of detachment. Low affect alone does not constitute a disorder.