The economy is booming, take-home pay is rising and millennials are getting married and having children. Despite all those homebuying catalysts, real-estate agents said this could be one of the weakest spring selling seasons in recent years.

The culprits: rising mortgage rates, a new tax law that reduces the incentives for homeownership and a growing weariness among first-time buyers being priced out of the market—all of which are expected to damp demand for homes this year.

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