Henrich et al. remind us, like others before (e.g., Bornstein Reference Bornstein and Bornstein1980; Graham Reference Graham1992; Kennedy et al. Reference Kennedy, Scheirer and Rogers1984; Moghaddam Reference Moghaddam1987; Parke Reference Parke2000; Russell Reference Russell1984; Sexton & Misiak Reference Sexton and Misiak1984; Triandis Reference Triandis1980), about the formative role of culture in all human behavior. Even basic psychological processes such as perception are subject to cultural variation (Segall et al. Reference Segall, Campbell and Herskovits1966). Nonetheless, psychological research remains largely ethnocentric.

Consider basic processes in motor development. Cross-cultural comparisons serve as natural experiments revealing the effects of experience on motor development and highlighting diversity in developmental pathways and the range in human potential (Adolph et al. Reference Adolph, Karasik, Tamis-LeMonda and Bornstein2010; Bornstein Reference Bornstein1995). Yet, the field suffers from long-standing assumptions of universality based on norms established with WEIRD populations.

Historically, research on motor development focused on establishing universals. Led by Gesell, early pioneers established the practice of cataloging the ages and stages of motor development. In particular, Gesell's (Reference Gesell1928) testing procedures, test items, and developmental norms – explicitly and deliberately based on behaviors of WEIRD children – inspired the widely used Bayley (Reference Bayley1969) and Denver Scales (Frankenburg & Dodds Reference Frankenburg and Dodds1967), which describe the developmental timing and sequence of infants' motor skills. Such normative templates are the current, accepted gold standard of motor development, and are regarded as prescriptions of what is desired, rather than relatively narrow descriptions of what may be acquired.

Due to the prevailing emphasis on motor milestones, cross-cultural research has been dominated by normative comparisons of onset ages. Recent evidence shows that cultural differences in daily childrearing practices can explain accelerated and delayed onset ages relative to WEIRD norms (see Adolph et al. Reference Adolph, Karasik, Tamis-LeMonda and Bornstein2010, for a review). For example, in some regions of Africa, the Caribbean, and India, caregivers vigorously massage and exercise infants as part of daily bathing routines, stretching infants' limbs, tossing them into the air, and propping them into sitting and walking positions (Bril Reference Bril1988; Super Reference Super1976). Infants who receive massage and exercise begin sitting and walking at earlier ages than infants who do not (Hopkins & Westra Reference Hopkins and Westra1988). Laboratory experiments with random assignment to exercise and control groups confirm these results: A few minutes of daily exercise accelerates walking onset (Zelazo Reference Zelazo1983).

Reciprocally, restricted practice can delay the age at which children reach motor milestones. In Northern China, the practice of toileting infants by laying them on their backs in sandbags for most of the day delays the onset of sitting, crawling, and walking by several months (Mei Reference Mei, van Rossum and Laszlo1994). Among WEIRD families, the recent practice of putting infants to sleep on their backs rather than their stomachs has resulted in delayed onset of crawling and other prone skills (Davis et al. Reference Davis, Moon, Sachs and Ottolini1998). In cultures that do not encourage crawling (including WEIRD infants circa 1900), large proportions of infants skip crawling altogether (Hopkins & Westra Reference Hopkins and Westra1988), either bum-shuffling or proceeding straight to walking (Fox et al. Reference Fox, Palmer and Davies2002; Robson Reference Robson1984; Trettien Reference Trettien1900).

Other aspects of motor development are also influenced by culture and context. For example, childrearing practices can affect the shape of developmental trajectories. In WEIRD cultures, upright leg movements show a well-known U-shaped trajectory: Newborn stepping disappears after about 2 months of age and upright stepping does not return until the end of the first year. But in cultures where caregivers exercise infants' leg movements (and this is confirmed in laboratory experiments), stepping shows monotonic increase throughout the first year (Super Reference Super1976; Zelazo Reference Zelazo1983).

Foot-binding in China provides an extreme example of how cultural practices affect the form of movements. For 1,000 years, mothers deformed their daughters' feet to give them the walking gait of a “tender young willow shoot in a spring breeze” (Chew Reference Chew2005). Feet 3 inches in length were achieved through years of training and excruciating pain. The routine (typically beginning between 5 and 8 years of age) involved breaking four toes on each foot and bending and tightening them in place with bandages. Girls then relearned how to walk with altered balance constraints of their shortened feet. This custom was eradicated in the 1920s.

Cultural practices also affect the endpoint of development. Daily tasks require peoples of Africa, Asia, and North America to develop walking and running skills that exceed the abilities of WEIRD adults. African women and Nepalese porters of both genders carry immense loads by modifying their walking gait to conserve mechanical energy (Heglund et al. Reference Heglund, Willems, Penta and Cavagna1995). They routinely carry more than their body weight for many kilometers (Bastien et al. Reference Bastien, Schepens, Willems and Heglund2005). Tarahumaran Indian children, women, and men of Mexico run 150 to 300 kilometers round-the-clock for fun and for persistence hunting (Bennett & Zingg Reference Bennett and Zingg1935). Endpoints can also stop short of what is expected. Crawling on hands and feet before walking is typical in WEIRD infants, but some families of adults in rural Turkey crawl on hands and feet instead of walking (Humphrey et al. Reference Humphrey, Skoyles and Keynes2005). In contrast to most cultures, the parents of these adult children never encouraged walking, and the primary models for locomotion were siblings who also crawled instead of walked.

Henrich et al. raise an important point about commonalities across cultures with different childrearing practices. Basic motor functions – manual, postural, and locomotor skills – that are universally useful and adaptive are present in every society studied. We are comparing the postural and manual capacities of 5-month-olds in disparate cultures on maternal handling practices (Karasik et al. Reference Karasik, Bornstein, Suwalsky, Zuckerman, Adolph and Tamis-LeMonda2010). Despite different support contexts, infants practice various postures with accompanying opportunities for object exploration. These data highlight developmental equifinality (Bornstein Reference Bornstein1995): Although the routes to object exploration vary, the outcome is the same.

Cross-cultural research on motor development is important for establishing general principles in developmental science and for revealing possibilities in human development hitherto unimagined. WEIRD infants sit at 6 months, but African infants sit at 4 months. WEIRD mothers would never dream of leaving their young infants unattended, but mothers in Cameroon leave their 5-month-olds (for 20+ minutes!) sitting alone on high stools. These sorts of phenomena can only be revealed with cross-cultural work providing the impetus for laboratory investigations to consider and test hypotheses previously not envisioned.