Wood moves as moisture levels change. Thus the desirre to have the wood acclimated on site prior to installation.
In general, summers are humid, winters are drier and less humid. If the wood was installed while it had a higher moisture content, then in the winter when the air is drier, the wood will shrink. Quartersawn will move less than flatsawn. Since it moves on a percentage basis, wider boards will move more than narrow boards on a per-board basis. A bad example, but for a very simplistic example, if the wood species shrinks 1/4" per foot of width, you'd have 1/4" gaps between boards if you had 12" wide flooring (adjacent boards would each shink 1/8" on each edge, creating a 1/4" gap between adjacent boards, If you used 3" wide flooring, you'd only have a 1/16th" gap. Again, a bad example just for argument's sake.
Engineered flooring, due to it's multi-ply design, is more stable and will move less than solid wood.
Things you can do? Like Randy wrote, control the interior humidity. Use a humidifier in the winter, and in the summer, whole-house air conditioning naturally dehumidifies. There's a balance in there somewhere.
Good luck!
