Imagine 5 small villages deep within the Indian sub-continent. Really, they are smaller than villages, think of each one as a cluster of huts that house little more than an extended family group.
Further, this part of the sub-continent is in a brittle part of the monsoon weather pattern.
Every farmer realizes that he needs a cow or an ox to plow his field to plant the field. The cow eats vegetation that is too bulky for humans to efficiently digest. The cow has a rumen which is an enormous (20 gallons) fermentation vat that is upstream of her true stomach. The vegetation is softened and nutrients are extracted by microbes in the rumen. Those nutrients are then available for her use.
She gives a little bit of milk. She poops, which makes acceptable fuel when dried. She makes calves at a rate that roughly replaces those that die. When she dies, a caste of untouchables comes and butchers her. Her meat disappears and her hide goes into the leather trade.
The monsoon weather pattern involves a dry season and a wet season. It is usually very orderly and predictable. The rains start in June and continue until September. Except when they don't.
"As in much of the tropics, monsoonal and other weather patterns in India can be wildly unstable: epochal droughts, floods, cyclones, and other natural disasters are sporadic "-from WikipediaImagine 20 years go by with solid monsoons and then a rainy season misses. Suppose one of the villages decides to eat their cattle. When the rains come the next year they are thrust into poverty by the need to buy replacement animals. They lose some choice parcels of land, or must sell the dowry they were saving for their daughter(s), who consequently cannot get married and remain a drain on the family. Perhaps they must migrate to the city where they become the poorest and most vulnerable class.
The other four villages watched and learned.
Then 100 years go by with solid monsoons. The lessons fade. Things are different now. Those lessons happened 5 generations back. Surely they no longer apply.
Then the monsoon misses for three years in a row (like in the movie Lagaan). Four of the five villages succumb to the temptation and eat their cattle. After eating their cattle, they realize they have no future on the land and abandon it. They joint the mass migration to the nearest urban area (like the Okies in the Dust Bowl years).
The remaining village then has the choice of all the abandoned fields when the rains return. Or, alternatively, the only dowry available to marry off the daughters from the villages that ate their cattle are their choicest fields. Either way, the village that conserved (some of) their cattle, the keystone technology in that economy, become dominant.
The point is that taboos that are irrational when times are good are often the only thing that keep you from being blown off the cliff during the storm.
Times are good most of the time. That is, good times are normal. Normal people are rational. Most of the evidence available to people are that taboos are expensive, embarrassing, clumsy, cultural expressions. Cool people are proud of their disdain for taboos and make a show of thumbing their noses at them.
The genetic endowment of a species is shaped less by the sunny averages the species encounters than by the extreme events that trim the skirts off of the bell-shaped curve. In a similar way, our cultural endowment is shaped by extreme events and we drift away from that endowment at our peril.
The reason I chose to describe the phenomena using a cluster of five villages is because taboos evolve in a mosaic. This same pattern of lesson-learned, fade, and then reinforced happened hundreds of times over, in thousands of separate locations. Villages, families, groups who were luke-warm were inexorably wrung out of society with Darwinian ruthlessness.
A final observation for this post
The critical difference between the village that survived and the ones that failed may not have been a matter of how devoutly they adhered to the taboo in the final months. The critical difference was likely that the village that survived ask this question every day for the previous 102 years, "What do I need to do so I will not find myself in a position where I will be tempted to eat my cow?"
They may have built a larger grain bin and sold less of their surplus. They may raised few chickens and ate fewer eggs. They may have chosen spouses of smaller stature, thereby having lower caloric maintenance requirements. They may have downsized their "herd" early in the drought event while prices were good so there was more fodder for the remaining animals (remember that milk). They may have supplemented their diets earlier on with less desirable "famine foods" to extend their stored rice.
In other words, they over-reacted based on the explicit data that was available to them (and available to their neighbors). The critical point is that they reacted appropriately when the explicit data was combined with the information embedded within the taboo.