David Lack

David Lack appears as the principal alternative to Wynne-Edwards in the book’s account of animal population regulation. His work on bird clutch size supplies a gene-centered explanation for why reproduction is limited: an individual does not need to restrain its births for the welfare of the group if producing more offspring than it can support results in fewer that survive to reproduce.

Identity and Place in the Argument

Lack is described as a leading ecologist whose research focused especially on clutch size in wild birds. The book calls him the chief architect of the selfish-gene theory of family planning, while noting that he would not have used that name for his own account. The significance of this framing is that Lack’s findings are made to serve a broader argument about selection: population regulation can arise from the reproductive outcomes of individuals without requiring animals to act for the good of a group.

His position contrasts with Wynne-Edwards’s proposal that animals deliberately reduce their birth rates to protect their populations. Both accounts can acknowledge that animal populations do not increase indefinitely and that animals regulate reproduction. Their disagreement concerns the explanation: Wynne-Edwards’s account makes restraint beneficial to the group, whereas Lack’s makes clutch size advantageous to the individual parent when it maximizes the number of offspring reared.

Clutch Size and the Costs of Raising Offspring

The evidence begins with variation among bird species. Gannets and guillemots typically incubate one egg at a time, swifts three, and great tits half a dozen or more. Individual variation occurs as well: some swifts lay two eggs, while great tits may lay twelve. The book treats clutch size as at least partly genetically influenced, while cautioning that the relevant variation is unlikely to reduce to a simple set of single-gene alternatives.

Lack’s central point is that more eggs do not automatically produce more surviving young. If producing and caring for offspring both require limited resources, an increase in the number of eggs can reduce the care available to each chick. For a swift, if three is the optimal clutch size, a parent attempting to rear four may end up with fewer surviving offspring than a more cautious rival. The optimum is therefore not simply the largest possible clutch; it depends on the balance between bearing offspring and caring for them under the conditions facing that species.

That balance is grounded in the work of reproduction. A mother invests food and energy in making eggs, and parents may also invest in building and protecting a nest, incubating eggs for weeks, and supplying food after hatching. A parent great tit brings an average of one item of food to the nest every thirty seconds of daylight. If a parent spreads its limited food and effort too thinly, it may rear fewer young than it would have with a smaller initial clutch. Lack’s account treats the resources available to an individual female or mated pair as the limiting factor that shapes the number of offspring they can raise.

Selection, Population Regulation, and Human Contrast

On Lack’s account, individuals that have too many offspring are penalized through the survival of their own young: fewer offspring reach adulthood, and the genes they carry consequently pass into the next generation in smaller numbers. Selection can thus favor a clutch size that makes effective use of parental resources, without any need for animals to limit reproduction in order to prevent the extinction of the population as a whole. The contrast is not between regulation and no regulation, but between group-benefiting restraint and individual reproductive outcomes.

The book extends this contrast to humans by describing the welfare state as a change in the conditions that limit family size. Where parents’ resources do not suffice, public support can keep children alive and healthy; the costs of having more children are therefore no longer borne in the same way by the individual family. In the book’s account, this differs from wild populations, where parents unable to support too many offspring leave fewer descendants. This comparison underscores the scope of Lack’s explanation while marking an important difference between human social institutions and the conditions under which the proposed mechanism operates in nature.