The "Slow-to-Warm-Up Temperament" is a personality style often observed in children. These individuals may initially react with caution or hesitation in new situations or around unfamiliar people. They typically need more time to adjust and feel comfortable before fully engaging. This temperament is one of the three main types identified by Alexander Thomas and Stella Chess in their research on child development. Children with this temperament may appear shy or reserved, but with patience and support, they can gradually become more outgoing and confident in new environments.