Last year, a study of Swedish census information suggested a 4 to 6-year age gap is best, but new research has found that in some circumstances a surprisingly large gap – 15 years – is the optimum. Martin Fieder at the University of Vienna and Susanne Huber of the University of Veterinary Medicine, also in Vienna, Austria, studied the Swedish data and found that a simple equation related the age difference of the parents to the number of offspring.