Which of the following types of relatives are the most genetically similar?

A. Survey persons over the age of 55: This is a correlational survey design; it can identify associations between exercise and cognition but cannot establish causation because of possible confounds and lack of experimental control.
B. Compare residents of active vs non-active retirement communities: This is a quasi-experimental / naturalistic comparison; it’s stronger than a simple survey but still vulnerable to selection effects and other confounds (not true random assignment).
C. Randomly assign adults to exercise or sedentary activity and test pre/post: This is a true experimental design (random assignment + control) with pre/post measures, which is the method required to infer cause-and-effect.
D. Case study of a 60-year-old competitive weightlifter: A single case study provides rich detail but has no generalizability and cannot demonstrate causation.
E. Compare a biking club to a book discussion group: This is an observational/comparative design subject to self-selection bias (active people join biking clubs), so it shows association but not causation.

A. The proximodistal principle:Proximodistal describes development from the center of the body outward (control of trunk before extremities), not specifically arms before legs.
B. The cephalocaudal principle:cephalocaudal means development proceeds head-to-toe/upper-to-lower, so control of upper body/arms tends to precede control of lower body/legs.
C. Schema theory:Schema theory concerns cognitive structures for organizing experience (Piaget), not the directional pattern of motor development described.
D. Synaptic pruning:Synaptic pruning is a neural process of eliminating excess synapses during development; it doesn’t name the body-to-limb sequence of motor control.