Chapter 02: The Art of Designing Objective Functions
Chapter 02: The Art of Designing Objective Functions
Authors: Angel Zhang & Charlie Cao
A platform’s objective function determines the entire system’s “soul.” It is not some mysterious document — it is a line of code, or more precisely, an optimization direction.
2.1 The Choice of Objective Function
Here are optimization targets that different platforms have used or currently use:
| Platform Type | Objective Function | Real-World Consequence |
|---|---|---|
| Early Facebook | Maximize likes | Content converges toward shallow and pleasing |
| Mid-era YouTube | Maximize watch time | Extreme content amplified |
| Short video platforms | Maximize completion rate | Content converges toward short and punchy |
| News recommendation | Maximize click-through rate | Clickbait and emotional content prevail |
These are not system bugs. They are the optimization results of the objective function.
2.2 Why “Cognitive Health” Is Not in the Objective Function
The reason is simple: it cannot be quantified.
- Time on site: measurable
- Click-through rate: measurable
- Return visit rate: measurable
- Cognitive diversity: hard to define, even harder to measure
- Long-term well-being: requires years before data emerges
Commercial systems must drive decisions with measurable metrics. The measurable gets prioritized; the unmeasurable is systematically ignored.
2.3 Iterating the Objective Function
When an objective function causes a PR crisis, platforms adjust it:
- After the 2016 election: Facebook tweaked its algorithm to reduce weight on “divisive content”
- After negative press: YouTube reduced autoplay of extreme content
- Under regulatory pressure: platforms began incorporating “health metrics”
But these changes are typically reactive adjustments under PR pressure, not proactive values-driven design.
2.4 Who Designs the Objective Function
Not a single evil individual — but:
- Product managers: define the core KPIs
- Data scientists: translate KPIs into optimizable loss functions
- Engineers: implement and deploy
- Leadership / investors: apply growth-data pressure
Each person is rationally doing their job. But the accumulation of these rational actions produces a systemically irrational outcome.
2.5 Chapter Summary
The objective function is not a technical problem. It is a values problem.
Whatever you optimize for, the system moves in that direction.
If “user cognitive health” is not in the objective, the system will never care about it.