This article argues that the introduction of a presumption that equal shared parental responsibility is in the best interests of children into the Family Law Act in 2006 has contributed to inappropriate, and even damaging, post-separation parenting arrangements for some children. The author suggests that the presumption and its legislative link to equal and substantially shared care time orders have created a ‘legoscience’ that shared parenting is almost always good for children, but this lego-science is a pseudo science which is not consistent with the complex reported social science about shared parenting. The foundation of the lego-science is the presumption, but expressions like ‘meaningful relationships’ contained in other sections build a legislative or ‘lego-bridge’ to the time provisions. This lego-bridge has been reinforced by the case law. This article argues that a presumption was an inappropriate legal tool to use in the discretionary culture of family law decision-making because it encourages a ‘one size fits all’ approach. Further, presumptions are legal fictions that become dangerous when believed. The fact that the reforms were driven by fathers’ rights groups provided a charged socio-political climate in which legal fictions were more likely to acquire the aura of truth. It also seems that the safeguards against the application of the presumption and the making of share care time orders were drafted in a manner that has allowed them to be ignored, creating a gap between the apparent legislative intent – to provide exceptions – and how the law actually plays out in the courts and the community – with the safeguards by-passed at times. The article concludes that fundamental reform of the Family Law Act is required again.