In the two-period model with investment, we are confident that the labor supply curve is upward sloping at any wage level because we are making which of the following assumptions:

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Question restatement: The query asks why, in the two-period model with investment, we are confident that the labor supply curve is upward sloping at any wage level, and it presents four possible assumptions about the relative sizes of substitution and income effects.
Option 1: 'The substitution effect of a change in wages always compensates the income effect.' If the substitution effect fully offsets the income effect, total hours would be unchanged in response to a wage change, implying a perfectly vertical or indifference response rather than an upward-sloping supply. This option therefore mischaracterizes the typical wage response when there is a trade-off between leisure and work.
Option 2: 'The substitution effect of a change in wages is always larger than the income effect.' When the wage rises, the opportunity cost of leisure increases, encouraging more work due to the substitution effect. If this effect dominates the income effect (which could encourage more leisure or more work depending on the sign, but in standard labor-leisure models with normal preferences and investment considerations, the substitution effect tends to dominate across a broad range of wages), the net response is more hours worked, yielding an upward-sloping labor supply. This aligns with the common comparative-static result used to justify an upward-sloping supply curve in such dynamic/dual-period frameworks.
Option 3: 'The substitution effect of a change in wages is always smaller than the income effect.' If the income effect outweighed the substitution effect, a higher wage could lead to fewer hours worked (you’d work less to enjoy more leisure despite the higher price of leisure). That would tend to produce a downward-sloping or ambiguous labor supply, which contradicts the stated confidence in an upward slope. Therefore this option contradicts the typical mechanism that yields an upward slope.
Option 4: 'There is no substitution effect.' A scenario with no substitution effect would imply hours supplied do not respond to the relative price of leisure vs. consumption, which is inconsistent with standard microeconomic theory of labor supply where the wage rate is the price of leisure. This would not generate an upward-sloping curve and is generally considered an unrealistic simplifying assumption in this context.
Synthesis across options: The core insight is that the wage acts as the slope-determining price between leisure and work. When the substitution effect dominates, higher wages incentivize more work to exploit the higher reward per unit of time, producing an upward-sloping labor supply. Conversely, if income effects dominated, the direction of hours could vary, and if they always cancel, there would be no response. The statement in Option 2 captures the conventional assumption that drives the upward slope in this model. 
Additional nuance: In a two-period investment model, agentsTrade off current vs. future consumption and investment returns; even here, the intuition that wages reallocate time between work and leisure (and between present and future) relies on substitution effects being prominent relative to income effects to maintain a positive relationship between wage and labor supply across wage levels. This is why Option 2 is the focal assumption that supports an upward-sloping labor supply in this setup.

In the two-period model with investment, we are confident that the labor supply curve is upward sloping at any wage level because we are making which of the following assumptions:

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