The purpose of the study was to show that the change from reaching wit
hout grasping to reaching with grasping during the first 6 months of l
ife carried the characteristics of a discontinuous phase transition (c
atastrophe). A cross-sectional study was carried out with 58 infants b
etween 60 and 408 days old. The infants were seated in a specially des
igned seat, and presented with nine detachable balls on a black curved
board within reaching distance al shoulder height. The number of reac
hes without and with grasping were scored from video. A cusp catastrop
he model was fitted to the data. A Likelihood-Ratio test indicated tha
t rite likelihood of the cusp model was significantly higher, p < .001
, than a linear regression model. The cusp model was also compared wit
h a logistic model. Akaike's Information criterion for the cusp catast
rophe exceeded the logistic model, thus indicating a general better-fi
t. Based on prior research, the following potential control parameters
were chosen: crown-heel length, total body weight, arm length, arm ci
rcumference, ponderal index, arm volume, arm weight, and body position
relative to the horizontal. The cusp model predicted that arm weight
and arm circumference significantly contributed to the control paramet
ers. It was found that these two variables had their largest contribut
ion to the asymmetry control parameters. (C) 1998 John Wiley & Sons, I
nc.