Actually, babyeating in the common pen isn’t even internally stable. Let’s take the assumptions of the situation as given:
There is intertribal extermination warfare. Larger tribes tend to win and grow. Even division of food among excessive numbers of offspring results in fewer surviving adults, and thus slower tribal population growth and more likely extermination.
All offspring are placed in a common pen.
Food placed in the common pen is automatically equally divided among those in the pen and adults cannot selectively provision.
Group selection has resulted in collective enforced babyeating to reduce offspring numbers (without regard for parentage of the offspring) in the common pen to the level that will maximize the number of surviving adults given the availability of food resources.
Individuals vary genetically in ways that affect their relative investment in producing offspring and in agricultural production to place into the common pen.
Under these circumstances, there will be intense selective pressure for individuals that put all their energy (after survival) into producing more offspring (which directly increase their reproductive fitness) rather than agricultural production (which is divided between their offspring and the offspring of the rest of the tribe). As more and more offspring are produced (in metabolically wasteful fashion) and less and less food is available, the tribe is on the path to extinction.
Groups that survive will be those in which social intelligence is used to punish (by death, devouring of offspring before they are placed in the pen, etc) those making low food contributions relative to offspring production. Remembering offspring production would be cognitively demanding, and only one side of the tradeoff needs to be measured, so we can guess that punishment of those making small food contributions would develop. This would force a homogenous level of reproductive effort, and group selection would push this level to the optimal tradeoff between agriculture and offspring production for group population growth, with just enough offspring to make optimal use of the food supply. This group is internally stable, and has much higher population growth than one wracked by commons problems, but it will also have no babyeating in the common pen.
Actually, babyeating in the common pen isn’t even internally stable. Let’s take the assumptions of the situation as given:
There is intertribal extermination warfare. Larger tribes tend to win and grow. Even division of food among excessive numbers of offspring results in fewer surviving adults, and thus slower tribal population growth and more likely extermination.
All offspring are placed in a common pen.
Food placed in the common pen is automatically equally divided among those in the pen and adults cannot selectively provision.
Group selection has resulted in collective enforced babyeating to reduce offspring numbers (without regard for parentage of the offspring) in the common pen to the level that will maximize the number of surviving adults given the availability of food resources.
Individuals vary genetically in ways that affect their relative investment in producing offspring and in agricultural production to place into the common pen.
Under these circumstances, there will be intense selective pressure for individuals that put all their energy (after survival) into producing more offspring (which directly increase their reproductive fitness) rather than agricultural production (which is divided between their offspring and the offspring of the rest of the tribe). As more and more offspring are produced (in metabolically wasteful fashion) and less and less food is available, the tribe is on the path to extinction.
Groups that survive will be those in which social intelligence is used to punish (by death, devouring of offspring before they are placed in the pen, etc) those making low food contributions relative to offspring production. Remembering offspring production would be cognitively demanding, and only one side of the tradeoff needs to be measured, so we can guess that punishment of those making small food contributions would develop. This would force a homogenous level of reproductive effort, and group selection would push this level to the optimal tradeoff between agriculture and offspring production for group population growth, with just enough offspring to make optimal use of the food supply. This group is internally stable, and has much higher population growth than one wracked by commons problems, but it will also have no babyeating in the common pen.