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First described by Bateson (1979), kittens have an important “sensitive” period when individual life experiences can have lasting effects on behavioral, neurological, and sensory development. The sensitive period for socialization to humans is thought to occur between the ages of two and seven weeks of age in kittens. In one series of studies, kittens handled by humans between the ages of 3 and 14 weeks were more likely to approach humans and accept human handling for a longer duration than kittens who were handled after 7 weeks or never handled before 14 weeks of age. Withholding handling until 7 weeks of age resulted in kittens who seemed unafraid of humans but did not choose to remain in proximity to humans after initial contact (Karsh and Turner 1988). Another study found that even if handling is delayed to 5 weeks of age, kittens can catch up by 6 months of age in terms of sociability with humans to those kittens for whom handling began several weeks earlier (Lowe and Bradshaw 2001, 2002). These studies indicate that exposure to gentle handling from humans by 7 weeks of age is critical for a cat to enjoy human contact.

Other studies have focused on the quality of human exposure during the sensitive socialization period. Kittens housed at rescue centers demonstrated significantly fewer signs of fear toward humans when handled for two and five minutes daily from birth to 45 days compared to those kittens who experienced only passive exposure to humans during basic husbandry, such as cage cleaning (Casey and Bradshaw 2008). Kittens handled five minutes daily from birth to 45 days of age were more likely to approach strange toys and unfamiliar people as well as slower to learn avoidance tasks (Wilson et al. 1965). The end of the socialization period (seven weeks) corresponds with further development of the emotional system and fear of novel stimuli. Intraspecific social development has received little research attention, but the timeline presumably parallels that of cat‐human social development (Bradshaw 2017).

The socialization development period also corresponds to a time of exponential physical development. Kittens begin running around five weeks of age and have full coordination by seven weeks (Peters 1983). Air righting, or the ability of cats to land on their feet after falling, is first observed between the third and sixth week of life. Kittens produce ultrasonic vocalizations, and the pitch becomes lower with age. Expansion of the vocalization repertoire and avoidance of agonistic vocalizations begins around four weeks of age. Interestingly, the vocalizations of deaf kittens are louder but otherwise almost identical to those of kittens with normal hearing (Houpt 2018).

Visual acuity and binocular vision continue to develop during this stage. Cats have been used extensively as models for research into the neurodevelopment of the mammalian visual system. From this research, we know proper sensory development requires environmental stimulation. Deprivation of certain visual stimuli during development creates permanent alterations and deficiencies in the visual cortex. For instance, cats raised in environments with only horizontal lines did not respond to vertical stripes because of degeneration of the neurons responsible for recognizing vertical edges (Blakemore and Cooper 1971). Kittens not allowed to visualize their front paws due to placement in dark conditions or in an Elizabethan collar did not develop fine motor skills needed for placement of the paws (Hein and Held 1967).