Early infant grasping involves radial fingers.

@article{Lantz1996EarlyIG,
  title={Early infant grasping involves radial fingers.},
  author={Chris Lantz and K Mel{\'e}n and Hans Forssberg},
  journal={Developmental medicine and child neurology},
  year={1996},
  volume={38 8},
  pages={668-74}
}
Halverson's hypothesis states that prehension gradually develops from ulnar to forefinger holding. This hypothesis was tested by measuring the grip force of individual fingers (digits II to V) in 36 healthy children from 2.5 to 7 months of age. The results dismissed Halverson's hypothesis, since it was shown that from an early age the index finger played a leading role in grasping, usually contacting and initiating the grasp prior to the other fingers and producing a stronger grip force.