The increased concordance in dizygous twins over full siblings shows that the in utero environment is important.
The zygote is dizygous, meaning two different eggs were fertilized by two different sperm.
If genetic factors are important, concordance in monozygous twins, who are 100-percent genetically identical, will be greater than in dizygous twins, who are only 50-percent genetically identical.
Monozygous and dizygous twins are difficult to distinguish at birth - finger prints and blood proteins are used.
The incidence of multiple births increases with the age of the mother but distinctly more so for dizygous rather than for monozygous twins and more so still for triplets.